Red Hat Software, Inc. has chosen to license its code under the GPL for two reasons. One is that this is the easiest thing to do since the Linux kernel is licensed this way and our code interacts with other GPL'd code sufficiently to make any other Licensing scheme difficult. The other is that for the purpose of cooperative development purposes we genuinely feel that the GPL is in fact the most effective license format.
The purpose of the GPL is much like any standard copyright license, to define what you can and cannot use the product for. However it differs substantially from commercial licenses because instead of restricting your use of the software its purpose is to guarantee your use of the software.
It in no way restricts the use of standard commercially licensed software with Linux or other GPL'd code. Only the changes that you make to the code that you ``borrowed'' from someone else that was licensed under the GPL must be made available to others under the same unrestricted rules defined by the GPL. Any product that simply uses or operates on top of Linux can be licensed under any commercial rules you chose.
There are many leading commercial software developers who are actively supporting their commercial products on Linux for which you must purchase a license, such as Motif from the OSF, Empress RDBMS from Empress Software, WordPerfect, and recently Applixware - the Office Suite from Applix Inc. and Red Hat Software. You will find a complete copy of the GPL in section I.