As with all offers like this Amazon are hoping that it causes people who weren’t subscribed before to stat paying them regularly, and posts like this are kind of giving them free advertising.

It’s similar to recommending something like reward credit cards (where you can make money off them as long as you pay them off in full and never pay any interest), if you’re sensible enough with your money then you can get a freebie but the provider is looking to profit off you messing up or forgetting.