I find that the shops with fabric weight info provided are usually one of two types. Those that are not limited to quilting fabrics aim at a much broader audience, and because weight is vitally important in other fabrics that they sell (nylon, wool, canvas, flannel, rayon, etc), they include it for their quilting cottons as well.
The other group, those that are selling hand dyed, hand printed, art fabrics, who have, perhaps, a much narrower audience, also include the weight information because it matters to their buyers.
There are, of course, a few quilting fabric shops who also include weight info, Favorite Fabrics most noticibly. I know of two others I have run across, but I can't for the life of me remember who they were. :oops: (I'll PM you if I'm ever able to reboot those brain cells, Nan.)