It's all a matter of incentives and expected gains, in which case there is no incentive to scheduling Troy, and there is no gain to be derived.
Economically, for the state of Alabama, it's a dumb argument. 92,000+ people are going to be packed into Bryant-Denny regardless of if we are playing Tennessee, Troy, Tulane, or Timbuktu Community College. The opponent isn't going to have any major effect, relatively speaking. Unless you are playing a team like USC, the marginal economic impact of a game against a mid-major program will be a constant.
And there is no real other gain to be derived, either. Oh sure people would be interested in an Alabama v. Troy match, but that goes for Alabama v. North Texas as well. This is a football mad state where people live and die with each snap, and adding Troy to the schedule doesn't really change that one way or the other.
So what gain is there to be had for UA? None.
However, there is a great deal of potential loss to be had in the long-run. In the short-term it wouldn't hurt us, really. Oh sure Troy may beat us once or so and their fans would go nuts, but big whoop. Long term, though, if the program keeps rising and rising and we keep playing each other, you have some serious concerns. If Troy, hypothetically speaking, kept moving up the ranks and finally got into a BCS conference, then they would be a legitimate contender just like Auburn is, and they could severely restrict our recruiting base.
And that's why we don't do anything with Troy. We have nothing to gain from it in the short-term or the long-term, but we have a hell of a lot to potentially lose in the long-term.