The Magnuson-Moss Warranty Act
The Magnuson-Moss Warranty Act is the United States federal law that governs product warranties. This law enacted in 1975 by the US Congress requires manufacturers and sellers of consumer products to provide consumers with detailed information about warranty coverage. In addition, it affects both the rights of consumers and the obligations of warrantors under written warranties.
Section 102 (c) of the Magnuson-Moss Warranty Act prohibits companies from including a tie-in sales provision in their warranty. Just as General Motors can not force consumers to purchase tires, gas or windshield wipers by refusing to honour the warranty on their cars neither can the manufacturers of ink-jet printers refuse to honour the warranty on their equipment simply because you chose an alternate ink supplier.
Below is an except from the The Federal Trade Commission (FTC) web site.
"Question: Will using 'aftermarket' parts void my warranty?
Answer: No. An 'aftermarket' part is a part made by a company other than the vehicle manufacturer or the original equipment manufacturer. Simply using an aftermarket part does not void your warranty. The Magnuson-Moss Warranty Act makes it illegal for companies to void your warranty or deny coverage under the warranty simply because you used an aftermarket part... "
Source of the above quote.
Other links
Guide to Federal Warranty Law