1) Your iPad, purchased in the US with a carrier selection, comes from the factory with a SIM card in it. This can be used to activate a prepaid data plan _on the device_ right out of the box - via that same carrier's network. (They are minimally provisioned before you get it.)
2) The Sprint/VZW iPads also support CDMA for 3G in addition to LTE (4G) and HSPA+ (GSM 3G). The "ATT" ones do not. (They are different hardware models.)
3) They are all unlocked, GSM-wise.