I bought an HP printer that came with an HP Instant Ink subscription a year ago. The subscription promises to send you ink when you're running low as long as you print within the designated number of pages.
I recently changed my card and figured I would let the subscription expire.
Fast forward to today. I go to print something and find that the printer is "unable to print" even though there is ample ink left in the cartridges. I press a button on the printer and it spits out a report that states the printer is unable to print, except for printer reports (!).
I dig a little (since the error message they show provides no additional information beyond not being able to print) and find this thread  in their support forum. It turns out that once the subscription is cancelled or suspended, you are no longer able to use the ink that has been sent to you. Some even report not being able to print with cartridges they bought independently.
It turns out that their terms state that you're buying the ability to print x pages and the ink is actually always owned by HP, even when in your possession.
This has to be one shadiest and just overall worst product experiences I've come across in a while.
Printers have always been a bit of a pain but since when did they have to be near permanently connected to the internet else threaten to cut you off from all of their capabilities.
I do agree that it is a bit wasteful but unfortunately it isn't economical for them to retrieve the partially-used cartridges from cancelled subscriptions, so it is just thrown away. It would be interesting if they offered a "buy out" option. When you cancel the subscription with half an ink cartridge they could sell it to you for half of the price.
If they allowed use of cartridges after the subscription expiry then the system could easily be abused by only subscribing for one month at a time to refill your cartridge then cancelling until you actually used it up. There are workarounds for this like minimum subscription length or blocking people based on address but they have other problems.
The real shady shit is rejecting third-party cartridges, that should be illegal. It's your printer and you should be able to decide what ink you use.