Sony didn't really have a choice. They have to protect the integrity of their system as best they can, which means closing any holes that are used to circumvent the copyright restrictions. Besides just being important to protect sales on their platform, it wouldn't surprise me if there aren't contractual obligations here as well.
That doesn't really have any bearing on Sony's contractual obligations as a console vendor. And you'll note PC games are often DRM-laden, and the DRM vendors surely have their own contractual obligations to do as much as they can to prevent their DRM platform from being compromised. Similarly, I know for sure that back when Apple sold DRM-laden music, they had contractual obligations to the music labels to fix any FairPlay holes they could.
They should be given kudos for even trying, not blame for failing at what appears to be an impossible task. Sony thought they could offer Linux-on-PS3 and still keep the PS3 secure. For a while it seemed to work, but eventually it was demonstrated that Linux-on-PS3 broke the security model of the PS3. Sony really had no choice, because it's a gaming console first and foremost and they had to protect that. Yeah it sucks for everyone who was interested in using it as a Linux machine, but if you can't recognize that Sony had an obligation to protect the gaming console over the Linux support, I don't really know what to say to you.