I believe that many different groups including vegans, conservative Christians, extreme right-libertarian militia organisations and various anarchist/communist groups have carried out attacks that have been characterised as terrorist in the past - and therefore that they could plausibly carry out attacks again, implicating 'fellow travelers' as they did so. I'm not sure about Occupy I'm afraid, as it didn't really take off in the UK.

I also think that the general demographics of the people who make up the establishment are pretty important when you consider who ends up being imprisoned without trial.

I think that we understand each other, but you've added some interesting points and examples that I hadn't thought of.