They have the right to protest, ofcourse. I just don't agree with it, and they're trying to disallow other students to engage with or to listen to Greer in this particular manner.
I probably would agree with the protest if they talked about her being paid for it, but it seems the focus is on the actual performative act of lecturing.
My personal inclination would probably be let her give her talk too, but I wouldn't necessarily impose that view on anybody else if they didn't want to give her a platform. Certainly I wouldn't bother going either way.
Honestly this whole thing is a determination the University and the students then need to make, not Dawkins, which is why his "put up or shut up; I know what's good for students" attitude just rubs me the wrong fucking way.