Hmm, in theory it's very good. In practice, SU's (which are separate from Uni's but will probably be expected to follow the same guidelines) tend to have anti-extremist policies for a reason.
Having the EDL or Islamic hate preachers hold events on site, that often are aimed towards externals, not students, would make me uncomfortable and could create massive backlash for the elected or employed officials that signed off on them.
Sinn Fein are not the IRA pisacake, they're a legitimate political party. But your reaction is a fairly good example of how context is key.