Would a bit more charity towards one's colleagues be such a bad thing?
Does going to a pig roast to, say, sell sodas make you not veg? Is being a member of the Ag club worse than actually eating the milk and eggs? What about if you go because you're interested in organic farming of plants? I know I go to steakhouses fairly often when I'm on the road, as we've got some here where I can reliably get a good potato bar, lots of veggies, and a good salad bar. It's someplace I can conveniently get a vegan meal (or at least close) in the land of No Such Thing As A Veg Restaurant. Yeah, you've got to watch out they don't put ham in the beans and such, but some places it's quite workable.
I don't know the people you're talking about, perhaps they are total posers. Definitely I get being annoyed at people who claim to be veg but eat meat. There comes a point though that we have to recognize that people who are mostly veg are mostly with us. Why do they attract so much more anger than those who make no effort at all?
It sometimes seems to me like a schism between similar religious groups, where people are at each other's throats over what appear to be minor points of doctrine.