I'm not sure they've been shown to be violent (unless you consider damage to property as violence- I know some do, but personally my "things are just things" stance limits violence to actions which impact people, who matter.
Broadly speaking though, I agree. What they did was criminal damage, undoubtedly, I have no problem arresting and prosecuting people for that. But I don't believe that it's terrorism, nor that it would have been so unpopular had it not been bloody embarrassing for the armed forces. Honestly, bolt cutters and some paint should not be grounding some of your air defence.
One member did very violently attack a police officer:
> A police sergeant was left unable to drive, shower or dress herself after a Palestine Action activist allegedly hit her with a sledgehammer during a break-in at an Israeli defence firm's UK site, a trial has heard.
https://www.bbc.co.uk/news/articles/c4g54g1r15eo
Of course, one violent member does not make an organisation into a terrorist organisation. But, just as a matter of fact, there has been some actual violence against a person.
https://www.bbc.co.uk/news/articles/c1dzq41n4l9o
> Giving evidence earlier, he said the group's only intention was to "break in, cause as much damage to the factory as possible, destroy weapons and prevent the factory from reopening".
I count "causing as much damage as possible" to be violent.
While I think graffiti taggers "damage property" but are non-violent. But in many places, rival gangs blow up/set alight/demolish their rivals' homes/businesses/vehicles, etc. That counts as pretty strong violence to me, even if no people are injured.
Anyway, talking of people being injured, watch a member of Palestine Action (Samuel Corner, 23, Oxford University graduate) drive a sledgehammer into a police seargent while she's trying to arrest his comrade:
https://www.bbc.co.uk/news/articles/c4g54g1r15eo
Full video, sledgehammer attack at 3m05s to 3m10s: https://www.youtube.com/watch?v=l6P7p_5D4hw
I'd designate them as a terrorist group for destroying factories, not so much for spraypainting planes. But I'd still support your right to say you support them, even though I'd disagree.