In order to understand that, you have to realise that veganism is not a diet, but an ethical belief. A huge part that often comes up is diet, of course, since we all eat, and often in a social setting. But it also concerns, for example:

Not using wool and leather

Not visiting for-profit zoos

Not using cosmetic tested on animals

Not riding horses or attending horse-related entertainment

It is an all-round ethical standpoint, and not just a diet fad. You may or may not agree with it, that's how beliefs work, but ridiculing the thought of it being a protected belief seems narrow minded.