It isn't just AT&T. The store reports that it isn't supported on the HTC One.
So even though you activated without a sim card and tried to download Google Wallet without being connected to the AT&T network ever, the Play store prevents you from installing the Wallet. So I guess we have to hope Google will certify this phone for Wallet. I think I will contact HTC after I get my phone (supposed to be tomorrow!)
Did you get the 25GB Dropbox storage though?