What law in the US makes it illegal to circumvent an encryption system? I can think of the DMCA's prohibitions against circumvention measures for DRM, but that's specific to protecting copyrighted works.

This FindLaw article http://employment.findlaw.com/workplace-privacy/privacy-in-t... agrees that employers have a right to monitor communications from their devices on their networks, especially when this policy has been clearly laid out and agreed to by employees. Expectation of privacy is a major deciding factor in US law.

I'm not sure of the legality of an ISP doing this. I would hope it's illegal, but ISPs are weirdly regulated compared to, say, phone companies.