Look up Jim Wright, please. His district was in Ft. Worth. AMR didn't move to Ft. Worth until 1979. Braniff was already here, on the west side of the airport.
Look up the
Wright Amendment, too. There were 3 commercial airline airports in the area. Ft. Worth had two, and had to shut them down to commercial airlines when Dallas and Ft. Worth agreed to build DFW back in the '60s. FTW was pissed that KDAL was still open. They lost the initial battle with the CAB (look that up, too, please) and the city of Dallas over SWA. After the industry was deregulated, SWA was not bound by the intra-Texas operational restrictions. So they went to KMSY. This again violated the agreement with Dallas. They sued again, but it appeared they were going to lose, so they got Jim Wright to help. AMR had little to nothing to do with these early flights. Braniff was probably more pissed off, since their largest hub was at DFW and they were also heavily invested in it.
So, in summary, the Wright Amendment restricted SWA from growing at KDAL. It did not help it. If anything, it helped Braniff, AMR an DAL, but we all know how THAT turned out? Or do we? Read Hard Landing - Petzinger. That's a whole other enchilade. It also helped out the city of Dallas in terms of revenue.