I only said “mainly” because of all the frum vegetarian advocates who’s stuff I’ve read, there’s just one whose motives I’m not so sure about (and some people don’t even consider him Orthodox so maybe he shouldn’t count).
Your “proba” would mean vegetarianism is completely assur. In fact, the Sdei Chemed said it’s assur to criticize vegetarians. Several well-known Orthodox rabbis, including the Nazir of Jerusalem (one of Rav Kook’s main students), his son the current chief rabbi of Haifa, the former chief rabbi of Ireland David Rosen, Rabbi Shlomo Goren, and the Kamenitzer Maggid, have been vegetarian. (Rav Kook was in favor of vegetarianism but in practice ate meat on Shabbos and Yom Tov.)
The vast majority of liberals and leftists want nothing to do with vegetarianism or animal welfare activism. Browse through their magazines and they never mention the subject. And some conservatives are certainly vegetarian and vegan.
In fact, the conservative author Matthew Scully, who used to write speeches for President G.W. Bush, recently wrote a lengthy article in favor of vegetarianism for the National Review. In any case, we shouldn’t denounce someone’s stringency just because we suspect it is connected to some political ideas we don’t agree with.