Step one would probably be making this practice illegal in the first place, which, as far as I can tell, it isn't. Putting the cart before the horse to worry about who's liable for doing something legal.
Not my practice area, and I don't know all the facts. But if they sold printers and later disable those printers, it doesn't strike me as unreasonable to treat that as a fraud or swindle in violation of 18 U.S.C. § 1341, or as an unfair or deceptive trade practice under 15 U.S.C. § 45.
If this were anything other than tech - if, say, IKEA sold you a bed frame that disintegrated the moment you used a non-IKEA mattress or comforter - I don't think the government would be so blasé about it.
(EDIT: Perhaps less of a case if the printer merely won't work with those cartridges rather than actually being disabled.)
At the end of the day I'm not sure that a lock that prevents you from using non-authorized equipment/refills/whatever is very different from established practice. If they bricked the printer altogether sure, that'd be a new frontier, and it seems like many commenters have erroneously understood that to be the story, but that's not what's going on.
