While it's certainly annoying to have someone claim the moral high ground for a completely subjective and arbitrary reason, it's hardly restricted to vegans/vegetarians. Anybody that wants to be smug will find a reason to do so, usually by gravitating towards a practice that they believe makes them superior to others; the result is supremely twattish people adopting vegan practices, as opposed to a normal person becoming vegan and adopting a twattish attitude as a result.
This is also very evident in religion, as well. Your average believer will usually just be a normal person that has accepted a certain explanation for life's big questions so they can focus on more productive things; they won't tell you that you're going to hell if they see you eating meat on Friday, or see you violating some other harmless tenet of their religion, because religion doesn't intrinsically transform people into twats. Now, the assholes you see picketing a soldier's funeral, or standing on a college campus shouting that all non-believers will burn in the flames of hell? They weren't nice, polite people before they heard a sermon that convinced them that the world must be purged of heretics... they were always twats, and when given an excuse to gush their twattiness all over the place (the right to religious expression, in this example), they took it and ran with it, twatting it up for all it was worth, using "their cause" as a shield against criticism for twattiness and tarnishing its reputation severely in the process.
Basically, your grievance seems to be with a particular excuse that some twats use to be openly twattish, as opposed to the twats themselves. A paradigm shift may be in order, here.
