c:\ProgramData is the replacement for AllUser AppData. It has become the default installation folder for all kinds of content, exactly because it is unsecured.
Other games put 50GB of content into user \ roaming \ appdata, either because they haven't noticed AllUser, or because they want a different 50GB for every user. Putting 50GB into 'roaming' even I can't explain.
The problem is that for users, stuff that wants admin permission is another irritating popup. Unless they know enough to be scared of admin requests, in which case it's even worse. So developers look for someplace to drop content, and they find places where they /are/ permitted to drop content, and it's OK, because if it wasn't OK, then the OS wouldn't let you do that, right?