Buying out of teaching duties vs. all duties using grant money
A university doesn't "obtain" funds through buyout -- rather, it frees up salary money that it would otherwise have to spend if the research funds were not in place.

What a university is allowed to do with such funds likely has to do with where the money came from.  In many cases, such funds will be unencumbered, and they can do whatever they want with them.

There are certain kinds of funds that might need to be used to support research, like (in theory) indirect funds from a granting agency.  "Support research" can have broad meaning, such as pay library staff, or keep the rooms where research goes on heated.

I'm not sure I understand the whole situation, the way you've expressed it, but it seems like it gives the university more freedom to creatively release people from responsibilities, and it doesn't set up any alarms for me.