Re: But *Why* did they buy them?
As far as I can tell, it is be part of IBM's cloud (or hybrid cloud) strategy. RH have become/are becoming increasingly successful in this arena.
If I was being cynical, I would also say that it will enable IBM to put the RH brand and appropriate open source soundbites on the table for deal-making and sales with or without the RH workforce and philosophy. Also, RH's subscription-base must figure greatly here - a list of perfect customers ripe for "upselling".