Gmail users sending to each other will only relay inside Google's own private network. If all of my co-conspirators are using Gmail, there are no external relays to be tapped. Someone would have to read all of our SSL/TLS traffic to see what we're writing about.
This is even more complicated when the data centers are in other countries, and none of the data actually enters the US. So if two EU users were accessing Gmail from the EU, the data may never enter the US at all. This means any network tapping would have to be done in the EU as well, requiring cooperation from many international telecom companies.
It's still easiest to just force Google to hand it over via NSL. Google's still legally bound to deliver the data even if it isn't physically stored in the US.
This is even more complicated when the data centers are in other countries, and none of the data actually enters the US. So if two EU users were accessing Gmail from the EU, the data may never enter the US at all. This means any network tapping would have to be done in the EU as well, requiring cooperation from many international telecom companies.
It's still easiest to just force Google to hand it over via NSL. Google's still legally bound to deliver the data even if it isn't physically stored in the US.