Conservatives believe in the rule of law. Try not to confuse my position with that of a Libertarian.
When it comes down to it, it's about the infringement of personal liberties.
I don't want to raise my family in a society that's inundated with stoned losers. I feel their " right " to pickle their brains on a daily basis infringes upon my rights to live in a society where my family and I are not surrounded by people under the influence.