It's obvious you don't know much about the airline business South of the border. This is high revenue flying. These customers want assigned seats, food, and they don't want to ride with rif raf. You'll see no flip flops and tank tops apart from the US customers you bring in. Remember too that it didn't take passengers long to figure out all they had to do was buy two tickets and they basically got around the meat of the WA. It'll be no different at HOU. And the passengers have to go through C&BP anyway. They will buy one ticket to HOU on Interjet, Avianca, TACA, LAN, COPA, etc. and buy another on SWA out of HOU. The only thing stopping it at the moment is you've only allowed one gate for them.
*Trust me, that scenario is going to play out. Unless you're giving away the International leg, the passenger is going to want to patronize the home team as much as possible. They won't hesitate to buy a cheap ticket thru the US on SWA, but as soon as you get them back to HOU they're going to want to get back on the airplane that feels like they are already home.