Is it because people who put them straight on the bike don't get that you may have to invert pads to fit front spec on rear (or rear spec on front)??
I suspect so, Etienne! I've not seen anything particularly different between the two except for pad/pad holder orientation. By placement behind the fork blades, my Nomad uses two "rear" brakes.
There are some exceptions, however. For example, the Scott/Pedersen self-energizing cantilevers used om some of my bikes have distinct "front" and "rear" models; otherwise, the helix responsible for the self-energizing action would be the wrong way 'round.
Best,
Dan.