I think it's twofold. First, we have an expectation that you arrive early and stay until the last second has ticked off the clock. Obviously, there are a small number of those who want to leave early if we are way ahead to beat the traffic, but by and large, people stay and participate. It's self-policing that starts at the student level and carries on throughout life. If someone leaves the game early in my section, I might say, "Hey doctor, best of luck to you with the medical emergency you must be leaving to take care of." Nothing mean to the person, but just reminding them of our social norms. You might also get called a two-percenter, a term used to describe the small nnumber of our student body who does not embody the Aggie Spirit. To us, sitting down or leaving early (unless medically necessary) at a game is akin to farting loudly in public. Sure, you can do it, but those around you are going to view you as strange. You just have to take the choice of not being a true fan completely off of the table by establishing norms.
Second, our fan base, especially the older guys, has seen the power of the 12th Man in action. Normally, it is our 30,000+ students creating the noise on every defensive play, but the former students know the importance of being loud on crucial plays. When you've literally seen an opponent false start, forget the snap count, be unable to communicate, or have to call a time out due to the crowd noise, you realize you can be an active participant in the outcome of the game and are not yelling simply for your own gratification. Everyone also realizes that it is a group effort and they are needed to participate.
Video Link: http://youtu.be/4NUxFE2oJ-M