It isn't blocked it just can't activate from a new sim as the IMEI isn't recognized. The sim would need to be activated prior to using an ESN that is registered on their network already.
No.
Google included an OEM partition so that carriers can customize the boot screen. There is also an ATT logo on the back. For all intents and purposes it is still a Nexus and there is no real difference between carrier branded devices and those purchased from Moto or GP.
Symantics having to do with the word "blocked"? What's the difference between IMEI, MEID, and IMEID?
Are you then saying that there are carriers where you can activate their SIM on their system and then use the phone with that SIM on other carriers?
Ouch, I would never buy an N6 from AT&T because of that desecration. I love my pure, unadulterated Android.