Most zoos receive some public funding from tax dollars, if I am not mistaken. Meaning, if it is not appropriate to erect a cross or nativity scene there, it shouldn't be appropriate to have statues of Buddha. I think we all know, if someone of Christian faith had put up a statue of Jesus in the zoo, there would be anti-Christian protests on those grounds, and to pretend that wouldn't be the case is foolish beyond belief.
Personally, I don't have a problem with ANY religious monument on ANY public grounds. I believe the Constitution gives us the protected rights to be able to do that, and not the contrary. It is the ESTABLISHMENT of religion, which congress is prohibited from endorsing, not the general acknowledgment, respect or reverence FOR religion. BUT.. if we are going to set the standard as we have, the Buddha's need to come down.