You're right, though there is some profit in shutting down computers. Even right now there's a proliferation of ransomware[0] [1] that locks out machines. I've removed several of those from computers in last two years and even once been paid to do this.

Moreover, if computer-locking DRM malware becomes commonplace, a market for cheaper-than-police unlocking will emerge, with incentives favouring hacking the DRM to then earn money on fixing it.

[0] - http://en.wikipedia.org/wiki/Ransomware_(malware)

[1] - http://en.wikipedia.org/wiki/Rogue_security_software