Even if they do it for ethical reasons they have the respect of others not to succumb to petty name calling and quasi aggressive character defamation. They accept that the non vegans aren't "enlightened" and just carry on with their virtuous life in private.
Just like I don't want to be preached to by a religious person or a politician etc... I'll listen, I'll acknowledge but I'm entitled to ignore or disagree. Now you can do this pleasantly like I have been doing and Johnno hasn't been, or you can be obtuse and just have one sided rhetoric and to hell with everyone else. Which one do you think will more likely make people engage and listen?