[nilkn]

People generally react strongly (and negatively) to what they perceive to be irrational behavior in others. I think there's a strong human instinct to want to set the correct precedent for others and for future generations, which leads to this sort of reaction when we feel as if someone else is setting the wrong precedent. The (very implicit) fear might be that the GF trend will lead to many resources being wasted on something that doesn't matter (special GF products, restaurants, marketing materials, etc.).

However, with dietary trends there's a second and possibly more powerful factor, which is that those not following the trend feel as if they're being implicitly judged by those who are. This feeling is often completely incorrect, but it's there nonetheless for many people. Ironically, the feeling itself is irrational, yet it feeds into one's own distaste for another person's (perceived) irrationality.

[kaitai]

I think it's the implicit judgment that is more powerful. If you were to declare, "I'm a summer; I can't ever wear yellow!" no one would care. When you refuse someone's bread, though, that's a different question. Food has historically been a way to bond, in some particularly ritualistic ways. (We "break bread together", history has people eating from the same dish to symbolically and practically show trust, etc.)

Much of the current GF stuff is not that good, whether you're looking at environment, cost, culinary quality, or health benefits. Why eat a bar of tapioca starch? It tastes terrible. And my delicious almond-flour brownies are destroying the aquifers of California even though they give me a delightful 300+ calories per serving. On the other hand, the proliferation of GF goods allows people with real problems to sit down with family and friend and eat crappy sandwiches together without anyone feeling judged by the presence of a lettuce wrap, and that has its own benefits. Irrational, or not? Yes.