There usefull for checking system stabilty if your building and selling pc's, save's you installing windows xp/vista just to check your system works, Thats what i use it for anyway .
Actually, it's most likely not illegal. Anything under GPL is allowed to be sold by anyone who wants to sell it. If someone is ignorant enough to buy something they can obtain for free, either it's their problem, or they have a legitimate reason (like not being able to download stuff).
I have old Mandrake, Debian, and Ubuntu CD's. I just keep them around to laugh at and remind me to stick with Windows and never go back.