The ATT Wifi Hotspots that work with your iPhones account do NOT roll over to the WiFi iPads.
I Install ATT WiFi hotspots and have also spoke on the phone regarding this very issue with their IT Staff. The only way hotspots from ATT will work with the iPad is either using an existing account or purchasing time or as others have said using the rewards program through StarBucks. For McDonalds they have it open for any Wifi Device.
Some Campuses also have ATT WiFi and it's the same as Starbucks, you need ATT prior access or purchase time on the spot.
Coupons are another means and sometimes just by calling their support line on the welcome page and a good enough story might get you one for the day. Or places will hand them out for guests during special promotions.