"AT&T, Verizon Before they agreed to install the system on their networks, some of the five major Internet companies -- AT&T Inc. (T), Verizon Communications Inc (VZ)., Sprint Nextel Corp. (S), Level 3 Communications Inc (LVLT). and CenturyLink Inc (CTL). -- asked for guarantees that they wouldn’t be held liable under U.S. wiretap laws. Those companies that asked received a letter signed by the U.S. attorney general…
I don't understand how the executive branch can guaranty immunity from civil action in the judicial branch.
The letters from the AG are protection from criminal action, and probably government civil action, that extend beyond the term of the administration issuing them (wiretap laws have criminal as well as civil provisions, and there are cases where the government can bring civil prosecutions.) For criminal laws, ignorance of the law is not a defense, but reasonable reliance on an interpretation provided by the public authority responsible for enforcing the law usually is a defense. For civil actions, reliance on the representation of the party bringing the action likewise can be a defense.
For civil action by a third party, unless there is a specific provision that makes this a defense for the particular offense at issue (which there may be, but I'm not aware of one), I don't think this would be particularly useful under any generally applicable principal.