What is the incentive to the manufacturer to do so? There are some specialty devices, mostly for law enforcement, that are essentially WORM SD cards. Evidence, a photograph most often, is written to it but cannot be modified. These command a serious premium.

Write blockers are pretty common, and not too expensive. The major issue is that writeblockers lag behind drive technology. Not sure how we'll write block the first Thunderbolt hard drives other than through software.