if att email is powered by yahoo shouldn't I be able to log in with my yahoo email?

I was trying to log into att because i wanted to try out disney go, but when I try to log in yahoo's domain is not a selectable option and this image is what I find at the att.net screen's homepage. I was signed in through yahoo but logged out; but I'm wondering if yahoo powers att.net  email then why can't I sign in (literally) on att.net with my email from yahoo or in general on the att website?