They're breaking the law, they can grab hold of them and pull them to the ground - which I might add is exactly what the stewards did, with no qualms at all.
Did you not hear all the boos and whistles when the announcements were made then? Do you not think that the atmosphere got just a wee bit more hostile after that?
I just don't understand the point of having the police there if they aren't allowed to act. They certainly didn't put people off running on the pitch.