They are a terrorist organisation as per the laws/policies of the country.
I happen to agree with the last para. based upon what is in the public domain; however, it has been mentioned that there are non public informations that point to something more dangerous.
But that's why I specifically said to disregard views about the designation - the designation is the designation, any challenges to that should be via judicial process.
So I can see no issue with anyone supporting Palestine Action being arrested - they are supporting a terrorist organisation.
If they want to support the cause of PA - do so.
But no-one can complain about being arrested for supporting the group itself.
And, much like the suffragette movement (who almost certainly would be prescribed if around today - they planted bombs if I remember correctly), it may turn out that those people supporting Palestine Action end up on the right side of history and me on the wrong side.
Edit
en.m.wikipedia.org
The Suffragettes were terrorists - they even had the, *cough*, balls to call themselves terrorists!