Bob,
I raised that same concern regarding US fleet working AA metal in PHX on this board, and how I found it to be offensive as AA FSAs were furloughed from PHX. For the record, I still feel that way.
However, there wasn't any "back room deal"... there wasn't any need for such secretive shenanigans, but rather simple legal maneuvering. Management has the contractual authority to bid FSAs to various other aircraft operations as a contractor, as we have handled British Airways, Allegiant, Continental, etc. ground operations in the past.
AA was using ATS as its contractor during the bankruptcy, and could have even requested US to bid for services during that time then too (although I have no idea if it was actually offered). So during the merger process AA Management agreed to use US FSAs with US Management approval as a contractor, until there is one fleet service work group. As financially they are one company, US could offer its services for a very low price which "saved" AA a lot of money, which in reality is just a wash. If the combined Management thought having AA operations on the US side of the airport, and using US FSAs as the best method, then a simple bid for services was the way without having to hire back AA FSAs.
Trust me... there are a lot of US agents who were furloughed out of LAS about 4 years ago, and they would love to swap positions with AA PHX agents based in LAS, when the time comes to have a single union. Since we are on the topic, what's the LAS break down in terms of seniority of full-time and part-time and lowest hire date in each group?