This is not the case anymore.

In the past I have had great luck getting T+ through expedia, but my last cross country/transborder journey through expedia booked in P which was tango. Was not happy at all that I missed out on 2500+ miles!

you have to look closely at the fare rules and regulations which is hyper-linked (usually on the 2nd or 3rd page of the booking), and look closely at what the first letter in the Fare Code is.

The list of fare codes and what class they book into are listed under the AC FAQ stickied at the top of this forum.

But expect to see more T as they seem to be rolling this on all third party travel sites.