IBM is a big company. They can pay one their 300,000 employees less than one average support license to replace all the work he does for them indirectly, and if he's opensourcing his software, it's not like he can stop them from using it even if he is mad.

I'm not saying this is the morally right, "best" in the long term sense, or otherwise socially optimal choice, I'm just saying it's the standard IBM choice.