FTD:

"The other is to buy a cartridge at roughly 20-percent off through Lexmark’s “Return Program.” A customer who buys through the Return Program still owns the cartridge but, in exchange for the lower price, signs a contract agreeing to use it only once and to refrain from transferring the empty cartridge to anyone but Lexmark."

Note that because the contract is with the purchaser, and not with the refiller/reseller, Lexmark can only use the original purchaser. Which is why they turned to patent law in order to try to and stop the refiller/reseller.