The changing tformed results are expected behaviour. You're rotating the source so the target object will be in a different position relative to it's position and rotation.
In the exmple ..
If I were to change this to 0,0,10 then the point in world space would be 10 units in front of the target. Note that the Targets rotation would now come into play. By using 0,0,0 this ensures the point in world space is exactly where the Target entity is and the Targets rotation is irrelevant here.
Make sense?
In the exmple ..
tformpoint 0,0,0,Target, Source
If I were to change this to 0,0,10 then the point in world space would be 10 units in front of the target. Note that the Targets rotation would now come into play. By using 0,0,0 this ensures the point in world space is exactly where the Target entity is and the Targets rotation is irrelevant here.
Make sense?