Cops can arrest anyone for any reason. If it wasn't for speech, it'd be for public intoxication or accusations of being a paedophiles or for potentially possessing a weapon.
Like the linked article states: the law doesn't permit the police to do what they do. Even if you implement an America-style "you can even yell bomb in an airport" speech law, the cops would still arrest people to intimidate them. Changing the law does nothing when the police force is simply ignoring the law.
