Was on student union back in the day (ours wasn't like this, though
The SUB, even though it's on campus, will be under the jurisdiction of the Students' Union. We owned our building, leased space to the various businesses there (bar, coffee shop, book store, and so on), and made the decisions about what went on there. The university administration wouldn't likely be able to do anything about this, short of offering the CF space to hold their display on UVic property (as opposed to Student Union property).
The student union will have a constitution which will (likely) state that all board members must be full-time students. Ours did, anyway. The SU will then likely have paid staff (office manager, or something), and is certainly within their rights to hire people to research/speak about issues. Only the elected board members (ie full time students) would be able to vote on motions.
The cross over to the university administration would be that the President (or maybe Vice, one board member, anyway) would have a seat on the University Board of Governors, and the Student reps from each faculty would sit on their various faculty councils. So the university really has no say on what goes on in Student union, but the student union gets a voice in the running of the university. If the students at large don't like what the student union is doing then they have to stand up and hold their elected reps accountable (as they are doing), but the university won't step in and do anything. It's the students' union dues that pay to run the students' union, so they need to ensure it's done properly.
For the record - I am not defending what they did, just pointing out that it was by the books. Disgusting, really...