It's just a guess, but I suspect the percent of diseased cuttings buyers receive is about the same as the percent of tubers they receive that either rot or do not have eyes. I don't order many cuttings but I have never had problems with mildew or disease on any of them.
Selling/shipping cuttings is much more difficult than selling/shipping tubers. With cuttings a vendor needs to get the cuttings rooted in time for the ship date but not be too large(old) for the shipping material being used. You can pack tubers several days in advance of the ship date but cuttings have to be packed just before shipping. If tubers take a week to arrive it is not problem, a cutting that spends a week in the mail is never a good thing.
Brambleside cuttings were the best I've ever received, I hope it is worth all the headaches and they keep shipping cuttings for many years.