This also happened on The Simpsons, when they went to Itchy & Scratchy Land, were urged at the entrance that they would need plenty if I&S dollars and then, immediately on going in, every single stall had a huge sign saying "Itchy & Scratchy Dollars not accepted".
I agree with you, OP. It isn't, as PP have suggested, begrudging paying the money that goes towards positive school activities; it's just terribly misleading to advertise a 'no cash policy' (which seems quite pointless anyway) and then for cash to actually be required to buy food and drink - and then to not issue refunds, when they must have realised that people would have overbought tokens, to include food purchases.
Call me a cynic, if you will, but it almost sounds like this was their plan all along, in order to maximise takings.
Whether it was or not, it's absolutely not the way to keep parents on your side, nor to encourage them to bother coming along again next year.