Because - the police are the ones who are enforcing the shitty world that people live in and the dominant powers. The people, the activists, the ones who are campaigning for a better world to live in, they're trying to set themselves apart from the bullshit and misery that has the planet so fucked in the first place. Right?
So yes, the guy was a cop. Yes he doubtlessly informed on many an action and an activist. Yes, could have easily followed through with the power station bullshit charge; but decided he couldnt go through with it. Something in there showed some humanity.
Now the cops probably wont have him any more either.
Is the activist scene ready to bring him back in, listen to what he has to say, go through a long period of restorative justice, meeting and listening to all the people who felt betrayed, with talking it out and creating a better world - or are they just going to call him "scum" (language of the tabloid media IMHO) and tell him, mob vigilante justice style, he's cruising for a bruising if he shows up in eng-ger-land again?
It is worth a try to reach out to him. He may have lots to teach and tell about what the cops do undercover. Very little chances for the activists to ever know what happens in darkened offices of Whitehall and Scotland Yard.
Or are activists just happy to ape/mirror the ostracisation that happens in society today?