Picky aren't you
I don't see it that way tbh, buying food on the premises, gift shop stuff etc, is essential donating to the benefit of the zoo.
When SLWP did free winter entry we made a point of buying some drinks and buying a few bits to donate some extra revenue to them.
Looking at it that way makes me quite the philanthropist, with considerable donanations each month to my landlord, Shell, Lidl and the Dutch government