Unfortunately, I think [the shortage of healthy options at the Bronx Zoo and other food courts is] due to convenience. I bet it's much easier (and cheaper) to keep frozen hamburger meat than fruit/veggies/dairy. There's less of a chance the frozen stuff will go bad. As for the hamburger buns? No idea. I worry what kind of preservatives are in it to keep it "fresh."
A great point to bring up. It is true that frozen items with long shelf lives are preferred over foods that spoil in a matter of days.
That being said, here is some healthy fare that shouldn't cause any problems for the zoo's business model.
Fruit smoothies (using frozen berries -- no ice needed! -- and long shelf-like milk)
Soy patties (frozen, just like hamburgers)
Healthy stir-frys (frozen chicken breasts + frozen vegetables. Added bonus: brown rice, which has a long shelf life).
Whole wheat hamburger buns (most people can't tell the difference, and the extra fiber won't hurt them)
Small boxes of raisins and small bags of pre-sliced apples (just like Subway sells)
Plain unbuttered popcorn
Healthy energy bars (i.e.: Lara, Clif Nectar)