I guess you are both correct. AT&T is probably being extra cautious.

On a good note....I transferred some membership rewards points (I have a US issued Amex, though I live in Canada) to my marriott account last night at 9:00pm ET and, although they do not show on the marriott rewards web site, when I called marriott rewards' toll free number, I noticed that the points have already been transferred.

That was quick!