Are Zoo's Worried That If They Have To Put Killer Whales Back Into Their Natural Habitat That People Will See We Don't Need Zoo's Anymore?

Unfortunately it has been proven as in the case with free Willy, the other Orca that
they tried to release, that they can't survive when released.

I agree with what Keithold and Madmacstew said. Further, I don't think such concern for orcas is as much because they are kept in captivity as it is that they are used for public entertainment. That practice is more comparable to animals used in circuses than animals kept in zoos.

We will still need zoos to preserve the species