Actually, I don't believe people like Mandrake are violating the GPL. The reason is because there is no difference between paying for Mandrake and downloading it. Both versions include the exact same thing. The only difference, in this case, is that Mandrake provides the latest version of their software to Mandrake Club customers first and then to the rest of the world second. Since they are not forcing you to buy Mandrake, rather they are just making you wait an additional 30 days to get it for free. BTW, if you are looking for the free version of Mandrake Soft, check out
http://www.linuxiso.org or
http://www.linux-mandrake.com (linux-mandrake is the official download URL for their software. I don't see it published anywhere on mandrake.com though).
Now, if you want to talk about linux distro's who I feel ARE violating GPL, take a stroll over to the infamous Lindows.com, in which the only way to get it is with cash money.