Yeah, and it doesn't make any sense as they already have the pictures in the original quality stored on their servers already.
I think (but I am not sure) you can use iCloud Photo sharing even if you don't use iCloud Photo library, but that's a special case. Then you could limit the quality of those photos, I guess. But why degrade the quality of photos that you already have? Sharing doesn't use any extra space than non-sharing. They are in the cloud anyway.
I live in Canada and am born in the early 80s. 1 in 20 of my friends has a phone that isn't an iPhone, we often use iCloud photo sharing for albums but I don't know anyone who is using iCloud Photo Library, so not a special case in my circle
I think (but I am not sure) you can use iCloud Photo sharing even if you don't use iCloud Photo library, but that's a special case. Then you could limit the quality of those photos, I guess. But why degrade the quality of photos that you already have? Sharing doesn't use any extra space than non-sharing. They are in the cloud anyway.