I can't imagine not wanting to accommodate a guest's food choices. We do eat meat, but are pretty selective about food sources- within our financial limitations. I have friends who are vegetarian, and when I wasn't comfortable planning a good vegetarian meal, I'd always ask for suggestions- they were always willing to share ideas with me and it took a lot of the pressure off. I have a very food-avoidant kid and visiting places for dinner can be hard as people tend to think she's just being a brat and unwilling to try things. She's trying very hard and has made great progress, but if you put sauce on her spaghetti she's NOT going to eat it. I usually make sure to bring along something I know she'll be able to eat, even if it is just a loaf of homemade bread.
The OP's MIL sounds like she's creating an issue just for the sake of proving she is right. I'd be inclined to always bring along dish to share that I could eat. If she's rude enough to make an issue of it, I'd ask her to pass it along for seconds. :)