Muddiest point for 9/27/10

I have a question regarding Red Hat and Linux.  I know that one of the stipulations of using Linux is that if you create a beneficial change to the program  you need to make it available to others.  With Red Hat are they allowed to take Linux and change it for their clients and then charge them for the services.  I understand that they provide tech support and are allowing others to access their product, but does it go against the Linux creation principles to charge for the product?