You are correct that it's an "at will employment" issue, but the OP is also correct that with at will employment you are better off terminating someone for no cause than a cause that is not provably accurate. If you give a reason but can't prove it - or even worse, your reason can be proved wrong - you have opened up yourself to a wrongful termination lawsuit regardless of "protected class" (though that would make it even stronger).
As the (attorney) lecturer for the class on employment law that I took said: "this is an at will employment state: the employee or employer can terminate employment at will with or without reason. And the exceptions make up 98% of the law."
In this case, firing someone for an undisclosed email sent by Comcast that "summarizes" an interaction that the employee believes is incorrect completely opens them up to a lawsuit. And a lawsuit would definitely force disclosure of the email, so if his side of the story is accurate it could be interesting...