No you misunderstand my meaning.
proscribing makes it illegal to support or belong to a group , because that support makes it easier for the group to achieve its terror related aims (by fundraising, recruiting new members, publicity, PR, etc). I don't think anyone thinks the grannies getting arrested are about to smash up a military plane, but they form the base of the pyramid that enables these actions.
If a pensioner was marching under an ISIS flag, its not illegal because we assume they are about to commit some atrocity, it's illegal because their support helps that group - whose aims are to commit such atrocities.
And those supporters of PA or ISIS can still completely legally use their speech to criticise/boycott Israel, or call for a Islamic caliphate etc. They just can't support the groups whose explicit purposes are to bring about those things through violence.