BEWARE: knowing them, there's got to be some hidden hook here... if not several. If you are going to try this, scrutinize the fine print. I near fully doubt they are generously giving away a month without a hook.
Initial wild (completely uninformed) guesses: auto-renewal for high fees, locked phone, free* only if you then take a lengthy contract, permanent forehead tattoo of their logo so you are a mobile advertiser for life, 1+ years of indentured servitude, first born, 3 vital organs of their choosing, immortal soul, etc. All of the above? Some of those are jokes... hopefully. With AT&T, you just never know. 😉
Caveat emptor!