Callistomen, you have put your finger on it. It is a danger for any organisation that organises demonstrations and the larger the movement the greater the ease of infiltration. However peaceful ER is, and I have no doubts that it is peaceful. Neverthe less there has been violence and behaviour like that mentioned by Lemongrove at most of its demonstrations. probably initiated by extremists with, ultimately, very different aims.
The danger is not that we ban all dissent, and anyone who really thinks that we do is living in cloud cuckoo land. The danger is that we allow to flourish, without investigation, every populist movement that has what can best be described as a benign aim (global warming, animal protection, organic farming) and ignore the activitists within it, who use the cloak of respectability belonging to the group they belong to, to pursue a separate agenda of disruption and violence for quite separate political ends.