1.) drivers same =/= not same headphones. Housings, damping materials and other stuffs make differences. In personal experience, even after such mods HD555 is not really exactly same as HD595.
2.) Not only Sennheiser, but pretty much all of other companies do same....
Grado: SR60 to RS1 same drivers only slightly different damping and treatments.
Beyer: DT770/880/990 using all same drivers, but different housing and materials.
Sennheiser : HD580/HD600 case (only grill and cable are different.)
Drivers are just small part of headphones. Same drivers do not guarantee same sound (HD497, I heard, use same/similar drivers from HD650. But do they sound same? No. I heard them and they are completely different headphones.)
This housing and materials do play major role on overall headphones sound (like room treatments make a big difference on HT setup.)
P.S : But I think, for Sony Qualia 010 case, in which only differences with SA5000 are pads and cable, sometimes such practices are indeed ripoff.