Many (perhaps even most) countries ban public acts of support for what they consider to be terrorist organisations.

For example, the USA maintains a list of Foreign Terrorist Organizations, and penalities for providing support to those organisations are rather more severe than the in the UK.

Whether or not Palestine Action should be proscribed is a different matter - their most serious action to date has been simply damaging military aircraft. Personally, I'd tend to say that proscription is too harsh a response for property damage with no violence against a person.

I guess in the US that anyone doing something similar would simply have been killed, rather than their organisation being banned. That's not exactly great either!