I believe that it is called "thieving". It is done to help with even heating of the board during reflow. It also supposedly helps with warping of the boards during reflow.
I'm with you... I have always used solid ground pours and I NEVER used thermal reliefs for my RF designs. The manufacturing engineers used to screem bloody murder when I told them "no" to thermals. I even provided evidence that thermals effected the frequency response of the signal. They hated it, but thermals were NOT used. They always said that yields would suffer, but it turned out that this was not the case. Better profiles took care of most of the uneven heating issues during reflow.