It makes sense to have the option to move track automation with clips because you're essentially automating the audio of those clips specifically, but I don't think it would make sense for Sonar to move bus automation with track automation because a bus might be fed with multiple tracks. If bus automation is moved then it affects the sound of all tracks which feed that bus, so it would be crazy for Sonar to assume that this is what you intended. I guess they could include an option to link bus automation to track automation, but I think there would be far fewer cases in which users needed this behavior than there are cases in which users need to move track automation with clips. Adding options adds to the overall complexity of the program (and also increases the potential for a user to do things unwittingly) and so Cakewalk have to consider just how useful and popular such an option would be.
Besides, it's not that hard to move the bus automation manually.