It's important to remember that the headline refers to the group Palestine action, which has carried out vandalisation of historical pieces, occupations of property, destruction etc (see here). So it's a group that fits the UK definition of an extremist group.
Some political groups (or even branches of them) can be political cults trapping members with coercive control It's not always idealistic well meaning people. The court that decreed them an unlawful group will have seen a lot of info we didn't.
I think we need to look deeper than the headline