I once attended a seated dinner party, where one the guests proceeded to unpack some bagels and smoked fish as the meal was being served. I learned then that since devout Jews often bring food to gatherings hosted by goyim, since one can never be sure of the kosher state of the kitchen where the food is prepared.
Although this could be interpreted as a sign of disrespect for the hospitality being offered, from the point of view of the Jewish guests, they were simply being considerate, and removing from the hosts the pressure of needing to cater to a minority of their guests.
But what about a restaurant setting? Vegans, allergy sufferers, Muslims, or any other patrons with dietary restrictions present a major challenge to chefs in general, as they disrupt the initial planning of a well run kitchen. To wit, our selection of vegan establishments are so few in Houston that vegans don't have the luxury of choice. And vegans hoping to eat out are often subjected to questions like, "why would a vegan go to a BBQ restaurant?"
Because dining out is usually a social experience. People with dietary restrictions accompany friends and family to places where, while they may in the minority, they still seek to break bread in companionship. Restaurant owners and managers are well advised to perhaps accommodate people like my Jewish friends above, who come prepared with some packaged food as a gesture of acceptance that in this case, the food itself is not the focus, rather shared experience is.
Likewise, to my fellow diners with strict dietary requirements, we must accept that life is a bit unfair, and to reward restaurants that understand this situation with our patronage. And to please tip anyway, even if you bring your own food.