I think this opening scene is an interesting personal tick of John Carpenter. He's a guy who has stated that he went into filmmaking to make Westerns. He grew up watching Westerns -- many of his favorite films are Westerns (like the amazing
Rio Bravo). Part of that influence is his love for widescreen-cameras, which traditionally have been used to film landscapes and other "epic" scenery, but Carpenter adopted it for the horror/sci-fi genre.
This opening scene though, is a Western-staple. You have the hero wandering into the city alone (all that's missing is the horse) from a seemingly desolate landscape. He's an outsider -- with no prior relation to the city, and his arrival will fix what is wrong. The music that's playing is also Western-like with the soft guitar music (no idea how to describe music, sorry). It's a classical Western set-up.
The early exhanges between Nada and Keith David I think says a lot about the films theme.
Piper has this to say. "I just do some hard work and the chance will come. I belive in America. I follow the rules. Everybody get their own hard times these days."
Here he's expunging the classical "American-Dream". You work hard, stay straight, and eventually, your chance for prosperity will come. Nada thinks that his individual efforts will bring about goodness in the world.
As the premise of the film reveals though, that that's impossible. The system is rigged. Our entire civilization is being run by ghoul-looking aliens (obviously a stand-in for the establishment). You can't "work-hard and achieve your dreams" because the system is set-up to exploit you. It's not some sort of fair, meritorious competition. You "working hard and playing by the rules" will only serve to enrich the system, not benefit yourself or your peers.
There is a delightful bit of subtext to this fight (which is just as hilarious as everyone has stated).
Earlier in the film, Keith David had expunged the ideology that life is a battle for survival. We all have to fight like wolves for the meger scraps that are offered to us. This is his mindset -- survival through competition.
Think about it, the reasons for the battle are absurd. All Keith had to do to make Piper stop harassing him is to put on the damn glasses! That entire sequence could have been resolved in a matter of seconds.
Basically, I think the fight is a metaphor for how hard it is to make some people alert to societal issues. People want to bury their head in the sand. Especially those like David, who see themselves as being only out to save themselves, people that are diehard individualists and refuse to look at societal issues. They refuse to see structures, trends, communal issues that affect everyone -- only focusing on their own survival.
Of course, the problem with only focusing on your own survival is that
it doesn't solve issues. And said issues will exasperate until they reach a boiling point.
Maybe it's emotionally easier to have a narrow-focus, not care about more overarching stuff that goes on above your head. By only caring about yourself, you make your mindset very easy, and thus don't have to deal with the complexities of such issues.