I'm thinking that since the link referenced in the initial posting is for a swap it may be that the person wanted to be sure that the fabric was comparable for each contributor. Perhaps in previous swaps there was too much difference in the quality of fabrics sent. However, there have been a few times that I have come across a fabric in a quilt shop that was a bit thin. Perhaps it was ordered for the print, or some special thing. But usually quilt shops do carry high quality fabric. And the person writing it may have a personal philosophy that if we don't support the quilt shops they will close. I have found a great variety at JoAnn's so you do have to look at it.
But for personal preference I agree, buy where you can, or prefer.