[Mozilla's given up on threading, so new subject]
Anyway, when a company wants to run a copy of RHEL, they are likely to also want the support & quality assurance that comes with the real McCoy; they are accordingly willing to pay. And the advantage of GPL'ed software in this case is the openness, transparency, and (usually) stability of Linux. Also, with Linux there is no vendor lock-in and mandatory upgrade treadmill. Those qualitites are also worth paying for, if you are a serious company.
That pretty much sums up RH's business model ;-)
Indeed. And I think it's a good model, too. On one hand, you don't screw your customers for profit (like M$ does), and on the other hand, you have enough revenue that you can fund and support continued development of Linux. [1]
Evan