I realize that I maybe in the minority here, but I have read the email (it was posted on another site) and it is ambiguous or poorly worded (IMHO) as on at&t (i.e. running on the at&t network) is different than the branded at&t handset.
In fact a colleague with a Pure Edition Moto X received the very same email and took the survey, so either Moto is testing something related to at&t but not limited to branded Moto X's or Moto made a boo boo and sent out invites to a larger group than they intended. In fact there is a post on the Moto forums from a person who had a Pure Edition and according to his post uses it on at&t (he/she said time to time) and also got an invite.
Dan