You sign up for an annual subscription, paying monthly installments.
They give you the option to pay half of it out if you want to cancel, it's no different than a 12 month phone plan in places like Australia.
God I hate Twitter. People on their are so used to only reading 150 characters, that it's started affecting them in their daily life, that they're not reading the paid agreements they're signing up to.
Edit: This isn't defending Adobe. I was stuck in this agreement for a whole extra year than I wanted to be, because I didn't get a notification about it renewing (but that was again, my mistake). Fuck Adobe, they have shitty practices.