Re: Android 6.0 Marshmallow for the Moto X Pure Edition
There should no be an issue with any carrier holding back the update. Moto needs zero approval from the carriers (Moto has flat out said this) and there is not an ounce of carrier programming on the phones other then APN settings and Moto was giving out test drive updates that I am assuming the carriers have no way of stopping. In fact once you pull the sim card out the phone it has no clue what network you are on. So in theory even if the carrier could block the updates all you do is pull out the SIM card, connect to WiFi, update and you're good to go. AT&T's own download manager ins't even present on the phone so the phone goes though Moto to check for updates, not though a carrier check. Moto would have to hold back the update from IMEI's that AT&T says is on their network.
No idea why the Nexus 6 is different but AT&T sold their own carrier variant so there very well could have been some sort of agreement with Google or Moto that the unlocked N6's on AT&T could not get it before AT&T's own variant got it or any number of other things. It's not surprising Moto utterly ditched the carriers in every way; the carriers hold too much power and cause nothing but issues and cost extra money for cell phone companies. I'm willing to bet once the switch is flipped 6.0 flows out to everyone regardless of carrier.