Lexmark Int'l, Inc. v. Static Control Components, Inc.
387 F.3d 522 (6th Cir. 2005) - full text opinion
Ink cartridges and toner are often very expensive (in fact, printer cartridges account for 90 percent of the global printer market), and third party workarounds such as the one in this case are an attempt to lower prices for the consumer. Alternately, there's LaserMonks, a group of Cistercian monks who sell cartridges individually and at volume for much lower prices than retail, and whose proceeds go to fund charitable works. (And if all that printing makes you hungry, they maintain a list of other orders that sell a variety of foods, including fudge.)
The European Union has also passed a law that forbids printer manufacturers from installing chips in their cartridges that prevent consumers from refilling the ink instead of buying a new cartridge. The law comes into effect in 2006.