Think of it this way: to provide a monthly subscription, they can either send a new cartridge once per month with 100 pages in it (with all the shipping, plastic, production, etc costs associated with that), or they can ship one six times bigger a single time. That’s much better for the environment.

If they couldn’t turn off cartridges when subscriptions stop, they’d have to send less-filled cartridges. Otherwise, there’d be huge incentive to sign up for one month, get your big cartridge, and cancel.