They did that on purpose, though: Palestine Action has been banned on the basis of being a "terror organisation", this means that supporting them is a criminal offence. Knowing that, they purposedly propested by holding signs saying that they supported Palestine Action... and therefore they were arrested as expected (and really the police has no choice in such cases not to undermine the rule of law).

Note what the Court of Appeal said when ruling that the ban on terror grounds was legal:

[It was] "a fundamental mistake to overlook the fact that Palestine Action overtly promotes unlawful violence amounting to terrorism. It is not - as claimed - a direct action civil disobedience protest group like the suffragettes, operating transparently in the open. It is a covert organisation which operates with secret cells to avoid the detection and prosecution of those using violence to destroy property and cause injury. " [1]

[1] https://www.bbc.co.uk/news/articles/c4gy927jx88o

The suffragettes committed dozens of assaults and arsons and at least one deployment of an IED, which suggests bad faith in the interest of political orthodoxy by Lady Chief Justice Baroness Carr.