It’s not quite that bad (yet…) - it has reasonably clearly labelled subscription and non-subscription cartridges. So when you cancel the subscription, you just throw the non-subscription cartridge back in.
If you’re printing quite a lot, but not enough to justify a laser printer, then they can work out okay. The vast majority of people would indeed be wasting money though.
I’m in the same boat - zero ads in Sync for Lemmy until this last update. I just assumed the purchase from Sync for Reddit had carried over, but I guess either LJ changed his mind or it wasn’t intentional.