<< I thought that you have to buy the Redhat version. Am I wrong? >>
Yes. You can download any version of linux, even the distributions for them. Some may not offer an iso, but they do offer ftp installs, which is a nice way to help promote sales to keep these companies going without violating the GPL. The small distros that do not offer this only use free software that you can install seperately, so they do not break the GPL. But you are probably looking at the bigger name distros.