Is that a tip jar on the table? Even if not, just the association is enough for people to feel like they have to contribute financially for the food. While that isn’t an outrageous thought, for people who are tight on their finances, it might be an uncomfortable situation where they’d rather skip on the food than take it and not pay a share.
Well - I thought I’d mention that it could look like that - but of course it really depends on the personality / financial situation of your friends whether that might have influenced them