You appear to be ignoring the pertinent issue here, which is the fact that those being arrested can continue to demonstrate about what’s happening in Gaza, at virtually any time and place and in any numbers they choose to.
Instead they choose spend their Saturday afternoons holding a placard up, with the ubiquitous smug look on their face, supporting a group which was quite rightly proscribed as terrorist organisation. By doing so they willingly and knowingly associate themselves with people using threats and violence to influence government policy; the true definition of terrorism. And all entirely needlessly.
That’s why they’re being arrested, and so I’m afraid your point about not being able to ascribe the actions of some members to all is frankly bollocks, because they’re choosing to associate themselves with people engaging in terrorism.
That’s also why it’s more than fair to describe these people as clowns. Bored middle class types from the Muswell Hill Revolutionary Front, thinking they’re above the law and that they can do whatever they like. Fortunately they can’t and that’s why they should be prosecuted.