It's a precautionary thing, there's still a risk of infection from people visiting high traffic areas even when the attendance is capped. Aquariums especially may be vulnerable to this given that they're confined spaces that often don't allow for social distancing.
Social distancing didn't work worth a damn at Denver Zoo today except where it was enforced -- in the restaurant. Attendance was down, but everybody, especially children, crammed together in front of any exhibit where something was happening.
Despite saying all indoor exhibits were closed, the Tropical Discovery building was open around noon.
On the plus side I did notice more cleaning going on around the grounds. The bathrooms had that just been cleaned smell.