Although, outside of that scene, I haven't often ran into them, so my guess is that most people are referring to the internet zealots, who tend to be an issue for any political, social, or cultural group, no matter how radical or mainstream.
as for the internet - it's full of opinionated zealots no matter what the cause.
The fringe is specifically what I'm referencing. I'm not saying punk in general is vegan, or hardline, etc. But the proportion of people within punk who are, compared to the general populace, is obviously much higher.
And I disagree that the scene is only a little more veg friendly than most, I'd say it's much more, except maybe scenes like conscious hip hop or hippy festivals.
Of course, I was specifically involved in very politicized punk scenes, so YMMV.