Here's my opinion on this: every brand or company or store or what have you has their own target market. Clearly, A&F's target market is to certain sized women. There are other stores that cater to all sizes of women, and stores that only carry plus size clothing. Yes, maybe A&F should be carrying larger sizes, but do they have to carry them? No.
Personally, I believe that their products are overly expensive anyway, and the few items from A&F and Hollister that I have were either bought from a consignment store or the clearance rack. I mean, yeah, they have some really cute stuff sometimes, but I'm sure you could find clothes somewhere else that are not only carried in other sizes, but that are also less expensive.
And that's my opinion.