You definitely do pay for 2 months when you sign up for AT&T. Like most companies they pre-bill your base plan the month before, but since it's your first month they bill the first month and the pre-bill of the second month. Any of your extra usage will be billed after it along with the pre-bill for the next month (like if you go over on minutes or SMS).
BTW, Apple is a big companies these days too 😉