Perhaps you also don't understand why someone would need a car that goes faster than 65? Or why do people need alcohol? Cigarettes?
Compare the death stats and ask yourself where the real danger is.
I understand perfectly that we don't need cars that greatly exceed all speed limits. Keep in mind that in some places the speed limit is actually higher than 65 mph. Citing and fining people who exceed the speed limit is one way of reducing the number of violators.
The danger to others from cigarettes and alcohol is not comparable to that of automatic weapons. However, as has already happened, the punishment for driving while intoxicated has greatly increased in the past couple of decades. Likewise, because of new regulations regarding where someone can smoke have become much more strict, people suffering from secondhand smoke is reduced.
If people want to drink and smoke themselves to death, so be it. Medical insurance premiums are already more expensive for smokers, perhaps drinkers should be added too. I see no reason I should pay higher premiums to compensate for other folks high risk factors, particularly when they are a result of their personal choice.