Rear brakes take more abuse than the front do or ever will because of their position on the vehicle (all the road debris and water/snow/salt gets thrown up by the front wheels and hurtled to the rear...this is exacerbated when driving at highway speeds). It's the same reason why strut bolts which attach the bottom of the strut to it's corresponding control arm or knuckle are almost always seized in the back (my rear strut bolts are seized and I have worked on an Element here at work which had one seized as well) yet the front ones in most cases can be loosened with nothing more than a decent impact gun or a sturdy breaker bar.
It does sound a bit odd and your situation likely merits special attention, but if you have driven 21,000 miles without a rear brake service at all, there is the possibility the pads and/or sliders were stuck in their respective holders which often causes premature wear. Honda recommends the first brake service here in Canada to be performed at around the 16,000 km mark (maintenance minder systems usually request it be done between 12,000 and 16,000 kms depending on the driving condition of the vehicle).
Again, not saying you shouldn't be upset over this and if there is no more information you can aquire from the dealer you went to, next time definitely request to see the parts and ask for a more detailed diagnosis and repair explanation if you get a bad vibe from the situation. It is your right as a paying customer and the dealer's obligation to satisfy you in that regard.