I suspect that's because the front pads wear so much more rapidly than the rears. I think you must understand the German mentality by now. Very practical and frugal, but maybe not the best out there.
There are simply some things they do that makes no sense at all. In this case, sure the rears wear more slowly, but in the vast majority of cases the right
front pads wear at the same rate as the left
front pads. Given that there is some life after the sensor hits anyway, you really only need a sensor on one front wheel, so why not one front and one rear if you're only going to have two on the car?
Now the cynical side of me suspects one of two things. Either MB did this thinking "the rears will be inspected during an A or B service because everyone comes in anyway and if they are worn, the customer can receive the personal attention that comes with an outrageous quote" or they did this thinking "if those of lesser intelligence don't bring their cars to our dealers and someone thus fails to inspect the rear brakes, then they can also buy rotors"... Either of those scenarios takes into account the fact that the rear brakes wear much more slowly than the fronts, so as long as the A and B services are done timely, they'll be caught before they trash the rotors.
In any event, for twenty cents worth of wire they could have run one of the sensors off of the rear and avoided such nonsense, right? For that matter, maybe I'll splice one in the next time I do mine.
Take care and enjoy (figuring out) the ride,