> The reason you're paying is for the ink to be delivered
No, you're paying to be able to print up to 10 pages a month.
Thought experiment: If signing up for a subscription required an agreement to mail (postage-paid) the cartridge back to them when the subscription lapses, would that be legitimate?
> Thought experiment: If signing up for a subscription required an agreement to mail (postage-paid) the cartridge back to them when the subscription lapses, would that be legitimate?
If it was to refill the cartridges and send them to another subscription customer, yes. If it was just to make sure they end up in the trash instead of getting used, then no, that should be illegal.