A Jewish poster on another thread said they did challenge someone on a placard that was inappropriate last weekend and the protestor in questioned apologised and put it down. I'm Jewish myself and I'd like people to always intervene like that but I can't expect it of everyone, it can take quite a lot of social skill and confidence to do that and not everyone has it.
Regarding the protestor who was on the war memorial, my understanding is that he was asked to get down from it and he did. It's a bit disrespectful but I don't think it is something anyone should be arrested for personally.
I hope they do arrest people with hateful placards or who shouted violent slogans, or anything that supports Hamas. But we can't expect protest organisers to vet every attendee or their placards realistically, and we can't let the possibility of it happening shut down legitimate protest against Israel's actions.