"You receive a package to your preferred postal address, that contains the new ink and a sealable, labeled, post-stamped bag in which you will put the old ones and send it back to HP."

People seem to be missing this part. It's like returning a rental car.

You are technically obliged to return the cartridges contractually I believe. The fact that they don't pursue you if you don't is at least a small positive.

I can see why some think it's a dark pattern, but it's usual in a rental type scenario to return the thing that was on loan to you