They also have a substantial commitment to Linux and other free software technologies. If someone came after the Linux kernel with a patent infringement claim, IBM could choose to settle the claim by cross-licensing some of the patents in their vast portfolio, with no money changing hands. Such a deal could be consistent both with free software and patent practice. In most cases, economics weigh against nice patent licences for free software projects - in fact, I can't think of any such deals which have been successfully executed. But this is a separate issue from fundamental incompatibility.
Some companies avoid payments by swapping patents.
However, the wide-open cooperation of free software development makes it difficult, if not impossible, for the community to generate its own patents, says Eben Moglen, a law professor at Columbia University and the volunteer general counsel at the Free Software Foundation. The organization is responsible for the creation of the Gnu Public License. I can only hope Prof. Moglen was misquoted here. It's hard for me to see how cooperation makes it difficult to produce patents. Richard Stallman, one of the intellectual leaders of the open source movement, has been trying to organize a patent pool that would operate similarly to the Gnu Public License.