Vandalism and destruction of property is a shockingly low bar. The suffragettes threw an axe at the king and no-one said they should be a proscribed organisation.
I'll stand to be wrong, but I believe in one case a member of staff and two police officers were also assaulted. Terrorism isn't necessarily about body count, it's about motivation. If the motive is political change, and the ends is violence/criminal damaged/anti-social behaviour that tends to be enough. Similar cases exist in the US, too.
I personally think its a bit of a stretch and will likely be undone. However, to pretend they are simply peaceful protests being unfairly targeted is also incorrect.
Yeah the assault one is interesting cause they didn't end up charging them with it at all... perhaps reading the police statements without corroborating evidence is problematic.