I kind of get what you mean, actually.
So you have two friends. Friend 1 is veggie, Friend 2 is not.
When Friend 1 hosts, she cooks what she normally cooks, vegetarian food.
But when Friend 2 cooks, she cooks something she might be unfamiliar with, as she usually cooks with meat.
So actually on a practical level, if you just for a minute take out the moral aspect of it, Friend 2 is always hosting by cooking new and unfamiliar recipes. Friend 1, however, could be cooking a tried and trusted recipe.
Now I can easily see how that can get a little frustrating for Friend 2. I have a veggie friend, I'm not great at cooking, occasionally it's an added issue having to make something veggie (and interesting) as opposed to doing something I'm familiar with.
When I read the OP I have to admit I was in agreement, though that clearly puts me in a minority. However, of course everyone else is right that there is a moral, ethical issue here that you cannot ignore and you cannot expect people to overule.
But I do get the annoyance sometimes...