Its not that simple in reality because there's case law which applies.
That shows, as the Met have been repeatedly warned, that there is a near zero chance of successfully prosecuting someone merely for having a sign. You need to be able to demonstrate active support both for the proscribed group and that the offender is either directly taking part in terrorist activity or encouraging others to do so.
There's then the added complexity that yesterday there was an organised protest against the proscription of PA off the back of the government losing in court and PA being allowed to appeal the decision. You can legally protest against the act of proscription itself.
There's plenty of laws that get broken week in week out. We're repeatedly told there aren't the resources to deal with it. Try getting the police to come out if your car is stolen or someone tries to break in to your house. You are likely to be waiting days, if you ever see anyone at all.
So we know that the police have to take a decision over where to target their resources and the argument that some are making is that there is a disproportionate response being seen here and questions over who has instructed this to be the case around the country.
Yesterday the Met brought in officers from other forces and had to create a temporary holding facility as they don't have the facilities to hold the number of people they were hoping to arrest.
I'm not sure anyone can look at the footage of the protesters and think there was a grave threat if they were just allowed to sit there quietly for a few hours.