They didn't sell the ink to OP. With the subscription plan (which is entirely voluntary) You pay a small monthly fee for x number of pages per month. That fee is much lower than the cost of a cartridge of ink.

The downside is that you cannot use the cartridge if you cancel. Because otherwise everyone would subscribe for 1 month, pay maybe 1/8th of the price of a cartridge and then keep printing with it for a long time after.