True, but Apple would see a drop in the amount sold if they weren't priced in the ballpark of Android models. If the carrier cut the subsidy in half and you were faced with a $400 16GB iPhone 4S or a $200 16GB Epic 4G Touch, then $36 activation fee, screen protector, case, charger, etc. The extra $200 could come into play.
Maybe you are on to something with the amount Sprint needs to sell. Nobody knows how many iPhones AT&T and Verizon have agreed to sell, maybe all 3 are subsidizing to keep the iPhone moving so they can fulfill their contracts.