Um A good amount of buses actually do. Well school buses that is.
As far as cops giving tickets for not having a seatbelt on. Yea like someone else posted. There roads and rules.
I don't know what country you live in, but I know for a fact that school buses in the US don't have seat belts:
The fatal school bus crash yesterday in Arlington stoked a debate that has simmered across the nation for decades: Why don't school buses have seat belts?
http://www.washingtonpost.com/wp-dyn/articles/A63970-2005Apr18.html
The laws were established for your protection, if it was not against the law to wear your seatbelt, some people wouldn't bother use them. But because it is the law, more people wear seatbelts. It's also a source of revenue for many small towns adjacent to major high ways.
I know if it wasn't a law, that I'd still use a seat belt, because I know it would save my fucking life if I got into a car crash. All you have to do is educate people that seat belts save lives, and they will use a seat belt, not because they don't want to pay a small fine, but because they don't want to die.
You are going about this the wrong way. This is not an argument about how much the government loves you, it's about freedom. You're assuming that it's ok for the government to take away freedom if we get protection in return, this is horribly wrong, as stated by Benjamin Franklin:
"Those who would give up Essential Liberty to purchase a little Temporary Safety, deserve neither Liberty nor Safety." -Benjamin Franklin
You are giving up your freedom to make your own decision about wearing a seat belt, and in return you have the government make the decision for you, and take money from you if you don't obey them. We are slowly giving away our freedoms and responsibilities to the government, and letting them make all the decisions for us. The end result is totalitarianism.