Don't feed the animals

I took LLLL to the zoo last spring, and there we noted the following sign on the Monkey enclosure near the café:
Please do not feed us. We are on a special diet.

I didn’t realize that monkey diets were drastically different to humans ones, so I took a look at what they were eating:
Tomato, zucchini, melon, apple.
This didn’t look particularly special to me.
So I shrugged and went into the café.
I scanned the menu there: hot dogs, cheesecake, coffee, ice cream, meat pie, but not many vegetables. And I started to wonder.
Who was misfeeding whom?