You're comparing seat belt offenses to DUI? A drunk driver poses an increased risk to the drivers around him. A person not wearing his seat belt poses no additional direct risk.
We have laws against DUI for the same reason we have laws against speeding or reckless driving or even failure to signal--when an operator disregards these laws, there is an increased risk of danger for other motorists.
Actually, the so-called "laws against speeding" have nothing to do with increased risk of danger for anyone. Any statutory speed limit involving a posted speed that is lower than the design speed of the roadway in question is -- by definition -- a meaningless statute from the standpoint of public safety. The only purpose of these laws is to provide a mechanism for law enforcement officials to generate revenue.
The evidence I would offer in support of this is pretty clear. As a licensed professional engineer in my state, I am required by law -- at the time I renew my license every two years -- to report any convictions for felonies, misdemeanors, and even motor vehicle offenses. There are two very clear exceptions to the last item here: speeding tickets and parking violations. These two items are included as exceptions to the reporting requirements because a "conviction" under one of these offenses has no correlation whatsoever with my ability as a licensed professional to exercise sound judgment in the interests of public safety.
I'd also point out that we also have laws against the illegal use of firearms. That doesn't mean there should ever be a presumption on the part of law enforcement that someone carrying a firearm automatically represents an "increased risk" to others. The point that LetsRok raised is a good one. Unless a motorist is committing some kind of other offense, there is no way for this "increased risk" to be ascertained by a police officer without engaging in the kind of "random search" process that is clearly a violation of the Fourth Amendment.
So why do people get pulled over and cited for not wearing a seatbelt?