Nobody is making imitation Lexmark cartridges. These cartridges have already been bought from Lexmark, at which point they're the customers' property. If you bought a car, you can sell it to someone else, you can pay whoever you like to repair or modify it, you can paint it luminous green. Why should this be different?
Regarding your first point: the cartridges have been indeed bought. The customers at the end can refill them as they see fit. Instead, they sell them to Impression Products (it appears to be implied that they "recycle" spent cartridges they get from their customers), who refill them, and then buy them back.
So here the problem is not that the customer is not entitled to refilling the cartridges (which does not seem to be the point of the dispute), but the problem seems to be that Impression Products sells infringing cartridges (remember, not the content, but the plastic bit object of the patent) on which they make a profit in two ways: (i) they don't have to buy empty shells from a manufacturer, and (ii) they sell the product to a customer.
Regarding your car analogy: it's obvious you can repair your vehicle or paint it. This is akin more to a case of "customer not allowed to refill fuel tank". However, say that you want your car repainted, so the paint guy would strip the paint off your car, somehow liquefy it again, and then resell the paint as "compatible chevy color #8736". If GM had a patent on the specific pigment used, I can see how they could very well sue you, even if that paint had been used once before. It's all about selling a protected product, not about how many times it's been used.