Re: But *Why* did they buy them?
I'm fairly convinced that it's because of who uses RedHat. Certainly a lot of financial institutions do, they're in the market for commercial support (the OS cost itself is irrelevant). You can tell this by looking at the prices RedHat were charging for RedHat MRG - beloved by the high speed share traders. To say eye-watering, PER ANNUM too, is an understatement. You'd have to have got deep pockets before such prices became ignorable.
IBM is a business services company that just happens to make hardware and write OSes. RedHat has a lot of customers interested in business services. The ones I think who will be kicking themselves are Hewlett Packard (or whatever they're called these days).