Slot controlled airports are, by definition, unfair competitively. Anytime you artificially restrict the market place with something like a limited number of slots, competition isn't fair. SWA had to purchase AirTran just to get slots in DCA and LGA. Otherwise, getting slots there would have been prohibitively expensive, and would have taken years to accumulate enough of them to be cost effective, just like it took AirTran years to acquire all of our slots. Other carriers that could offer a better price to consumers couldn't get slots, while the legacy carriers were trading them around with each other to maximize their profits.
By contrast, there are no restrictions in Houston. SWA isn't looking for some sort of artificial controls on HOU that will give them a competitive advantage. If UAL wants to come over to HOU and fly a bunch of international flights on 737s just like SWA, then they're free to do so. The problem is that UAL does want a competitive advantage. They want artificial restrictions that don't belong in a free market.
Again, if you want regulation, just say so. I'm in support of that. But I'm not in support of partial regulation to benefit your company while screwing mine over. Free market or total regulation. Pick one.