No reason. As the Washington Post reports, you can use pocket-lining tricks like region-coding for all kinds of "content" -- including printer cartridges:

H-P has quietly begun implementing "region coding" for its highly lucrative print cartridges for some of its newest printers sold in Europe. Try putting a printer cartridge bought in the U.S. into a new H-P printer configured to use cartridges purchased in Europe and it won't work. Software in the printer determines the origin of the ink cartridge and whether it will accept it. ...

H-P is taking the same approach Hollywood has used with DVDs -- and one that prompted a huge consumer backlash overseas. Movies sold in the U.S., which generally are cheaper, are designed not to play in European or Asian DVD players.

The European Commission in Brussels has been scrutinizing such practices including DVD pricing "for quite some time, and we are still investigating," says Jonathan Todd, the European Commission's spokesman for antitrust policy.

Update (5:42 p.m.): Over at BoingBoing, an anonymous HP employee attempts an explanation; a reader responds.