It's the premise that once you give up data to a third party, you no longer have any "reasonable expectation of privacy" so therefore it's not a search.
There is similar case law backing up searching your garbage can. While it's up against your house (aka still under your control), you have rights to it.. once you put it on the street for the garbage truck to collect, you've surrendered those rights.
I'm NOT saying this is good or what I like - specifically, I hate it - but it's how it currently works under US case law.
Therefore, to change things, we need Congress to write new laws and/or new Amendments. Our opinions of "how it should be done" are irrelevant unless backed by the law.