Well, that is to say, this is the way Garmin decided to implement their software. Personally, I would prefer that sensors are linked to a profile. That would e.g. have the advantage that the speed sensor's circumference settings depend on the profile, i.e. if you move that sensor over to a different wheel, that corresponds to a different profile, you don't have to change the sensor settings. To obtain such a behaviour now, you'd need to have a speed sensor for each wheel - a good business for Garmin. Maybe that played a role in their design decision.
Changing their design a posteriori would certainly be a lot of work, so it won't happen, not even for the newer devices (also considering their commercial interest in selling more speed sensors).
You can always use the auto calibrate feature and not have to change any settings. Not quite as accurate as the measuring method but very close. I would think that there are very few people moving the wheel sensor to different wheels.
I use 2 bikes, and moving the cadence and speed sensors from bike to bike was a PITA. My sanity was worth the $70 for another set of sensors, I use my old 500 on the old bike and keep the 830 on the newer bike. When I want to ride, I don't want to spend time fussing with sensors and settings. Anyway happy riding.
