You make a good point that it is good for consumers to be informed. But it is also a good thing for sellers to be able to price discriminate between high-volume and low-volume users, which is made possible by making more profit on the ink than on the printer. If they could not do this then printers would be more expensive than most people would want to pay, even though the cost of production is actually less. So there would be a lost opportunity from gains from trade, which results in decreased total social welfare.

But to reiterate, yes it would be good if we all knew how much cartridges would cost up-front. I've even heard of some printer companies (Samsung, if memory serves) selling printers with half-full toner cartridges, to make the up-front cost even lower. This gives consumers the (false) impression that toner will be cheaper than it is, on the thinking that "I just bought printer + toner for $80, therefore toner obviously costs much less than this". Unfortunately, this isn't true when it's a half-full toner cartridge.