What you get from transmission data is the type, date, time, origin, destination, or termination of the communication. It does not include the content. What you get from a telephone is the time, the date, the origin, and where it went to.
I'm not seeing the gigantic difference that requires a higher level of proof, because you still have to get judicial authorization even on reasonable grounds to suspect. They have to go before a judge and convince a judge that they suspect a crime was committed before they get transmission data.