The source, environmental scientist Jennifer Jacquet, gives this as a litmus test for figuring out whether you're an "Eco-Douchebag". The comments thread is great fun; "Rob" there wonders whether dietary restrictions of this sort, that take on far more the air of religion than science, serve to enforce a separating equilibrium:
I've been saying for year that the more uptight kind of veganism (and even vegetarianism, if your talking about folks who won't throw their veggies on a grill that's had meat cooked on it. "That thar's CONTAMINATED carbon!") ... is an effective way to isolate its practitioners from the rest of the population, since they can't even sit down and eat with nonbelievers.The comments section goes on to point out that regulations require this kind of signage because organic foods that touch non-organic foods cannot be sold as organic. But a lot of other commentators seem genuinely offended to be considered an eco-douchebag for wanting to avoid bread that has touched other bread.
In the case of the eco-douchebags, the isolation helps them avoid interacting with folks who might question their dogma. They also get to feel more pure and saintly the more uptight they are about their ascetic trip.