That has nothing to do with it; tips is usually based on % of the value of service rendered, so tips for a 50-min massage is typically twice that of a 25-min massage, obviously.   Rather, they are hoping guests will upgrade their free 25-min massage by tagging on another 25 paid minutes (or increments of 25 minutes) to make it a longer massage.  Many guests do this.   Two free 25-min massages may turn into two 50-min massages (with half of each chargeable).  On the other hand