The short answer is that historically, we have treated documents in someone else's possession differently than documents in your possession. If the police are

The short answer is that historically, we have treated documents in someone else's possession differently than documents in your possession. If the police are investigating X, searching his house requires a warrant. However, going to his grocer and interrogating said grocer and getting records from said grocer has been less protected in the past. The grocer isn't the target of the investigation and so has less defense against being required to divulge information about the target. Problem now is that everything we do online has these intermediaries - other entities who hold lots of data about us - involved, and so "attacking the intermediary" has been exposed as a major privacy loophole.