Our guild also uses whatever fabric we can get, and much of it is donated by our generous members. Most of it that I've seen is nice quality - just stuff that someone had left over or decided they couldn't use. I can understand the notion that it is not nice to give less than decent quality, but there are many lesser fabrics that would serve the purpose of keeping someone warm. Some charities, I read on this board recently, actually encourage the donation of ugly quilts because, sadly, homeless people may be victimized by thieves if the quilt looks like something that might be sold for drug money.
I'm a little concerned that if a guild is tax exempt as a charity, perhaps it is not even ethical to promote certain businesses over some others. I'm not saying that can't make rules about the quality of fabric, but when it comes to where you should or should not buy it, that does not seem right, IMO.
If your guild's rules are not to your liking, perhaps you could organize a small group to do as you prefer. Someone will be happy to have whatever quilts you make.