On my campus, it's not rude to attend meetings just for the food. The food is there, as others have noted, to attract people to come. It's considered a fair trade - you show up for the meeting, whether or not you have an interest, we give you pizza. The hope if that if six students come just for the free pizza, one or two may end up actually interested.
As for the "starving students" trope, I think it's a kind of hyperbole that's just become accepted. Grad students have notoriously tight budgets, yes, but I've never known one who couldn't afford food. Maybe their food was Ramen, but they had it. But again, at my uni, this is deliberately played upon to attract people to meetings. Most meetings on my campus are advertised as: FREE PIZZA and a discussion about the history program. So, I don't see anything rude about showing up just for food, although it is rude to leave halfway through.