Then that is false advertising then. If they say the cases they are for the 6s, they are for the 6s!
Yup, I'd assume they're betting on the fact that nobody will bother suing them over a case that cost 99p. It's good logic I guess to sell them as cheap as possible, rather than having to dispose of it. I mean, it's really underhanded, but from a business point of view a 99p sale is better than 100% loss.