Don’t worry: where it isn’t, it will be, sooner or later @zoo_enthusiast...
I wouldn't call walk-through lemur exhibits a common feature in US zoos (I agree that they are quite common in Europe)... I've been to over 60 US institutions and only recall a very few walk through exhibits. Only San Diego Zoo Safari Park and a very small (and frequently non-operational) walk through in Philadelphia Zoo come to mind... There may be others, but still not really common.
Doesn't the Florida Aquarium also have a RTL walk-thru?
Walk-through primate enclosures are unfortunate rarer in North America, and my understanding is that the reason has to do with liability. In our highly litigious country, a zoo has to spend quite a bit of time and money to create a shield from lawsuits if someone gets bitten by a pissed-off monkey.