Now, my benchmark for whether a behavior is sane or insane is practical: 1) Does it cause a serious problem with everyday life? If not, it's sane. 2) If it causes a serious problem, is that problem greater or lesser than the problem quotient of life without said behavior? If greater, it's insane. If lesser -- and no other solution produces better results -- then it's sane. Sometimes the best you do is damage control and the results are never going to be actually "good" just "less worse."
With food, the basic goal is body maintenance. You need to eat enough, and the food needs to be safe and nourishing enough, to support the maximum level of health your body can attain. So if you're avoiding foods because they would make you sick, that's preferable to eating them, even if it throttles your diet down to nearly nothing. (This is, sadly, far more common than it used to be, as food allergies are skyrocketing.) If you're avoiding foods for philosophical rather than physical reasons, and you cut your diet down so far that it undermines your health, that's bad and you should change it.
A key problem today is the food supply is so trashed that it's easy to wipe out most of a supermarket with just one or two allergies. Seriously. We have a friend who's allergic to high-fructose corn syrup, and we're trying to minimize our consumption of it too. There goes about 95% of foods on the shelf, including some whole product categories. What's left are single-ingredient things like raw vegetables, raw meat if it's actually packaged pure, and staples like flour or milk. You have to read EVERY item on EVERY label because it's put in places it doesn't belong, like vanilla extract (which should have 2 ingredients: vanilla and alcohol). Gluten is another allergen that's in a huge number of foods. If you're sensitive to artificial colors, flavors, preservatives, etc. there goes almost everything except raw ingredients. If you're averse to genetically engineered foods or think you might have an allergy to them, you're down to buying organic and praying that there wasn't any contamination. And for any of those things, forget eating at a restaurant: it's a gamble, people are not careful, and if you do it then you will get sick sooner or later. Ditto eating at a friend's house unless said friend can be relied upon to be meticulous about your health needs (unlikely unless they have their own dietary issues and realize the importance). The lucky people just throw up or itch if they consume allergens; the unlucky wind up in the emergency room or dead.
So by the time you've thrown out the 95% or more of the food products that you can't eat, that's expensive and it looks crazy. It's better than being sick all the time, but dealing with other people bitching about your "eating problem" just makes things a thousand times worse. I have near-zero tolerance for that kind of harassment, not just of myself, but other people around me. Here's the bottom line: if a person's dietary choices make their health as good as it can be, that's fine, no matter how weird it looks. If their choices make their health worse, that's a problem and should be addressed. But unless you are their immediate family or health care provider, chances are you won't know which just from watching, and it's none of your business anyhow.