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.
Pipe dream, but I really wish we would make it illegal to use the terms "Buy" or "Own" for digital goods that can at some point not exist outside of your control.
I give you a dollar and get a DRM-free video file? That is buying.
I give you a dollar and can watch a video file an unlimited number of times in your app? That is not buying, and it should be fraud to claim that it is.