The analogy works because 99.9% of vegetarians don't even use words to directly tell others that they are unethical, they just assert their OWN vegetarianism as an ethical decision, and others go bonkers as though this is the most grotesque personal attack. This all happens without any red-paint-balloons or other extremist stuff.

In other words, when someone comes along and says what amounts to "for ethical reasons, I behave differently than you", people interpret it (understandably but unreasonably) as saying "you are an unethical person" (whereas the responsible reaction is, "hmm, maybe there's an ethical issue here that I should consider that I haven't yet"). Then, people focus all their attention on rationalizations to reject the other person, such as calling them "ideologues".

The Trisquel website doesn't say, "if you use or make proprietary software, you are evil". People react to the subtle implications as if that's what it said though. Maybe that's because people don't want to admit that they actually do act in ways that are ethical compromises?