Not much of a "bug" if it's taken years for people to notice because the function itself works exactly as expected.
If someone decides to have the send pan
follow the channel pan, then they would use the channel pan to adjust the send panning. The send pan becomes irrelevant, because you've already told SONAR's audio to do what the channel pan does.
Besides, tying them together graphically opens up a can of worms with respect to grouping because you can still have the pan send be part of a group. Follow channel pan overrides the group assignment (as expected) but does not alter it. If you were to remove following the channel pan, you would want the pan send setting to be defined by the group to which it belonged prior to setting it follow channel pan. Besides when listening with your eyes instead of your ears, you can always group the send pan and channel pan if you want them to move together. Following channel pan is a special case, and obviously the Bakers treated it as such.