I hear what y'all are saying about theming and its costs to the zoos, and rising gate fees, but if I may I'd like to offer another perspective.
When I was a kid, my home zoo, The Houston Zoo, was city-owned and was free. It was also a pretty lackluster zoo. By the time I was a teenager they started charging moderate gate fees, but it was really only in the last 15 years or so that the zoo really started to flourish, and though it now costs $22 for adults, it also has come to be considered one of the best zoos in the country. What changed? It was privatized in 2002, and they brought in Rick Barongi, formerly of Disney Animal Kingdom, as its director. He introduced a lot of the Disney-style theming, and it revitalized the public's interest in the zoo.
We have to remember that the bulk of any zoo's customers are not the diehard zoo fans like people on this site, they are families looking for a day of entertainment for their kids. Zoos are competing for those entertainment dollars with amusement parks, not just big ticket trips like Disney parks, but more local parks like Six Flags. By incorporating the theming that gives these families the theme park atmosphere and experience, zoos become more competitive with theme parks, and they get more traffic and can charge gate fees more in line with what the amusement parks charge since a themed-out zoo gives a better perception of entertainment value for the money than a traditional zoo, and that all means more money for the zoos. So when you talk about "money being spent that could've been invested into maintenance or improvements elsewhere" - well, no, that money wouldn't be there to invest into maintenance or improvements elsewhere, the money spent on theming is a capital investment made, often through loans or other financing, because it is expected to have a high ROI because it is going to attract more people to the zoo, ultimately yielding more money that can be spent both on maintenance and improvements and conservation than would be available otherwise.