Earlier quoted context omitted.
If you're an originalist (and I'm not saying that I am), data in transmit is not necessarily protected by that amendment If your data has left your house, is it really your data, or is it Google's say?
When you send a letter it does not belong to the post office while in transit. Why are bits on a hard drive any different?
You only have an expectation of privacy of the contents of the letter. You have no such expectation of anything on the envelope itself (e.g. sender/receiver address). The government contends that it only collects analogous metadata in transit, not the contents of the communication itself (although this is debatable).