It's more likely a matter of controlling group size. It's pretty simple for the staff to set a max capacity for the weight room and other indoor spaces and easily enforce it. Much more difficult to do outside unless you plan to park an employee in a chair on the field, just sitting there and watching. The department can also easily control access to the indoor facilities to keep non-approved people out; can't easily do that for the outside facilities. In other words, it's all about controlling access.