It's not that obvious at all, actually even under their question section they state the subscription is not required:
- Does an HP+ Printer require a subscription to the Instant Ink service or rother ongoing costs?
"No, there are no additional costs associated with HP+. You can choose to enroll in an optional subscription for Instant Ink and receive 6 months subscription, but it is not required."
If you paid money for a cartridge that doesn't work without a subscription, you got scammed by the seller, not by HP.
Instant Ink is an optional subscription, which they plainly state, and that costs money.
It is marked as HP+ eligible. It states that HP+ is chosen or not at setup (https://m.media-amazon.com/images/I/71OZXjots2S._AC_SX679_.j...)
In the requirements it says: "The optional cloud - connected HP+ printing system requires an HP account, ongoing internet connection, and use of Original HP Ink Cartridges for the life of the printer."
etc
I don't buy inkjets anyway because they aren't worth it over color laser printers, but i will say trying to understand any of this seems hard at a glance.