They're arrested for supporting a group that's been banned for causing around £30 million's worth of damage to our national defences at a time of hightened national security.
There's the implication that Palastinian Action are going to continue attacking us.
If they just stuck to protesting they would have been fine.
What gets classed as "support for" and "terrorism" is not evenly enforced.
These riots are spontaneous and "organised" via people getting riled up online. There isn't a central organisation that people see as leading these anti-migrant riots/attacks. They seem to be an emergent property of the protests. If there is a named group organising criminal action and it includes things that threaten/damage national security then that group should be banned.
Palestine Action was conducting organised criminal raids with the specific intent to cause damage to anything it felt was Israeli, Israel related, or somehow benefited Israel. A lot of the time the link was tenuous at best. They also attacked national security assets. Honestly this group's actions has done more harm than good for the Palestinian cause.