Others accused her of virtue-signalling.
"Pandering to the media?" she hit back. "I have worked with Oxfam almost exclusively on the rights and plight of women in developing countries. Women who have to supplement their factory/manual labour with sex work. There are NGOs who do not cover up the egregious abuses of their employees."
Yet as the scandal morphs and unfolds, no doubt other global charity behemoths will find themselves under the spotlight - and there may be more resignations to come.
Celebrity sells - a fact no one knows better than hard-pressed charities. But as celebrity ambassadors reach saturation point - Geri Halliwell joining the United Nations as an ambassador in 1998, fresh from the Spice Girls, may have been the nadir - it is worth asking, do they actually make any difference? And, at worst, could their role do more harm than good?
A recent paper published in the International Journal of Cultural Studies suggested the chief beneficiaries of association with good causes are the celebrities themselves, given that two thirds of those asked in two 1000-strong polls couldn't link a list of stars with the charities they supported.
"While awareness of major NGOs' brands was high, awareness of celebrity advocates for those brands was low," wrote Professor Dan Brockington of the University of Manchester and Professor Spensor Henson at the University of Sussex, authors of the research.
"Instead, it was plain from the focus groups that most people supported the charities that they supported because of personal connections in their lives and families, which made these causes important, not because of the celebrities. The evidence suggests that the ability of celebrity advocacy to reach people is limited, and dominated in Britain by some extremely prominent telethons and the work of a few stars."
The celebrity-charity symbiotic relationship stretches back decades. Hollywood star Danny Kaye became Unicef's first goodwill ambassador back in 1954, helping to raise the charity's profile into global consciousness. Diana, Princess of Wales was instrumental in raising awareness of Aids in the nineties; last year, Lou McGrath, the founder of an anti-landmine campaign group, credited the Princess' support with proving a "turning point" in the global effort to ban the devices.
Other celebrity ambassadors also stand out in particular: Angelina Jolie for the UN and Sir Roger Moore for Unicef.
Used judiciously, philanthropic star power is mighty. Who doesn't like seeing a famous name make a fool of themselves for a good cause? On a lesser scale, celebrities can give a voice to those who don't have one, and shine a light on often unfashionable, unglamorous and unpalatable topics.
Sometimes it's just downright heart-warming, such as actor Tom Hardy sharing with his 515,000 Instagram followers a pic of four Staffordshire puppies up for adoption from Battersea Dogs Home. Dogs + big-hearted hunks = a winning combination.
Lyndsay Cruz ran Oxfam America's public figures programme for more than 10 years and "absolutely adored our ambassadors during my time there", she says.
"Djimon Hounsou spoke at the UN during tough arms deal negotiations. Rooney Mara and Scarlett Johansson visited refugee camps and helped to bring awareness to the crisis in South Sudan and Somalia, just to name a few examples.
"These were dedicated yet very busy people placing their lives on pause to volunteer. So I am appalled and truly heartbroken to see what has unfolded at Oxfam."
Of course, while celebrity ambassadors can increase awareness of a charity's cause and message, they can also bring reputational risk. Scarlett Johansson quit Oxfam herself in 2014, following criticism over her decision to star in an advertising campaign for SodaStream, the fizzy drinks company that owned a factory in an Israeli settlement in the West Bank.
One talent liaison fixer who has worked securing celebrity names for high-profile charity campaigns explains: "You have no problem with the A-listers normally, they have no agenda to push and have done so well for themselves that they genuinely do want to give something back. It's the ones below that who are almost throwing themselves at you: they've been told it's good to tick the charity box, or who've had a load of bad press and need to improve their image.
"It's all very well becoming a patron for a charity but the bottom line is that it takes more these days than just posing on the red carpet wearing a branded T-shirt.
"No charity, big or small, wants to take the risk of being associated with someone who might say the wrong thing, or who gets involved in a scandal."
You can never tell how it might backfire on the well-intentioned. Last year, poor old Ed Sheeran was accused of "poverty porn" and reinforcing white saviour stereotypes by an aid watchdog for appearing in a film made in Liberia for Comic Relief.
On the flip side, his film was viewed more than six million times and formed part of a fundraising effort that eventually raised more than £80 million ($152.4m) for causes around the world - including helping children in Liberia. It is easy to sneer about "virtue-signalling" when a privileged celebrity gets on board, but when it comes to charity, money talks.
Celebrity ambassadors can also cost a charity dearly. Elishe Roche, one-time head of liaison at Oxfam, where she oversaw its team of ambassadors, has spoken out about the "horrifying misuse of donors' money", spent on high-profile "brand awareness" campaigns featuring said celebrity ambassadors.
Roche said that in this case the celebrities involved were "never the problem", but the money went on the circus that came with them: the expensive caterers and paid extras and glossy coffee-table books that no one looked at. In this case, perhaps, Oxfam bought into the celebrity myth more than the celebs themselves.
There are other uneasy details that often accompany some celebrity ambassadors: the private jets flying them around Third World countries, the minders and expensive hotel rooms, shelled out for by the charity and, ultimately, the donating public.
Sometimes, celebrities who want to do good just can't win. After a trip to Rwanda with the World Food Programme in 2013, pop star Christina Aguilera told US gossip magazine People: "This trip came at a time when I really needed to step away and connect with bigger issues in the world, this trip really touched me in a way I never felt before."
To her critics, it called to mind every gap-year traveller who has ever engaged in a spot of Instagram-friendly voluntourism.
Coldplay's Chris Martin, another Oxfam ambassador, is often singled out as the gold standard, for donating 10 per cent of his earnings every year, coughing up for his own flights on goodwill missions - and getting the teas in while he's at it.
But the real stars are the people who work day in, day out, with little support, in challenging conditions - and on far less than a celebrity salary - for a cause that they believe in.
"I almost feel as if all our good work will somehow be erased by the coverup and lies of some truly awful people - none of which I knew anything about," says Cruz. "It saddens me that the beneficiaries of generous donors might bear the burden in this."
