Spyware companies have sued companies that declare their software spyware. I don't think any have won, but it is possible.
Also it is important enough that Microsoft was pushing a bill to provide immunity to them if they put in tools that remove spyware. http://news.com.com/Microsoft+seeks+protection+from+spyware+firms/2100-7350_3-5718370.html?tag=nefd.top
The link is a little old so I don't know if anything actually passed. But this whole, copywrite, patent, DRM, DMCA thing is out of control.
(Sorry for the double post)
I think that the real issue with Antivirus companies not doing anything about the Sony rootkit is fear of lawsuits over DMCA. There have been several people sued and even jailed for breaking DMCA. Removing the rootkits, especially automtically like a good antivirus program should do, would seem to be a clear violation of DMCA. Sony will not sue at this point because of public opinion, but I think that this is a legal grey area and they could still sue if they were holding the line on the rootkit. An under reported issue is what the DMCA really means to all of this. Potentially it would still be illegal to remove the rootkit, if Sony wasn't make an uninstall avilable (their bad uninstall, not withstanding.)

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