Measuring the learning value of zoos is incredibly hard. The tendency is to measure plain knowledge - "Did you know that ..." type questions before and after a visit. What we are really looking for are positive changes in behavior and attitude towards animals and the environment rather than specific learning outcomes, which is much harder to attain. And in the main, it's not the zoos' fault. Incredibly dedicated zoo educators and increasing resources are funneled into providing sophisticated zoo education programs, but a visit to a zoo on a sunny day, lolly and balloon in hand, is so far removed from the real world problems that the animals in these institutions face, it is easy to see how hard the task is.
Add on the fact that many zoos still offer elephant rides, petting and other interactive opportunities with supposedly wild animals and it is ultimately a confusing and mixed message that is being given. These zoos may not be part of the accredited zoo community (although certainly some are) but the public don't know or care about that. They see all zoos as the same. We can estimate that approximately 500 of the 8,000 plus (publicly accessible) zoos that espouse acceptable / high welfare standards and are trying to continually improve. But because people generally don't or can't differentiate between this group of zoos and the thousands of extremely poor zoos, the perceived and real problem continues. This results in confusing the educational messages that the better zoos are so desperately trying to deliver. For example, mixed messages has resulted in many people still believe white tigers are a rare and endangered species, not an inbred anomaly, or that elephant rides are an acceptable performance in zoos.