That's the part people forget about "utilities" and "carriers". The reason they were indemnified, is precisely because they know the exact identities, (and even locations), of all of their users. The cops can handle things themselves, they just ask the "utility" or "carrier" who X is? And who is X connected, (or even connecting), to.
We have to think up an entirely different model for the newer technologies we are using.
This is not entirely true for at least the postal service - someone can address a bomb or drugs with some stamps and a fake sender address quite easily. Only the recipient address is known, but again people sometimes have mail sent to a known empty house to pick up.
This is very important to not forget. We might also note, though, that that same indemnification exists in a context where e.g. the postal service is under no obligation to keep people from sending things in code to each other, even if police would prefer all communications to be in cleartext.
We have to think up an entirely different model for the newer technologies we are using.