I know a lot of people that smoke weed that work at my store. They do it in their own time (after work) and wouldn't come to work high. No different than someone who enjoys a drink after work. Doesn't mean they will come to work drunk. If your a responsible enough adult I don't see the difference between someone who drinks or smokes. And before anyone argues smoking is bad for you, so is drinking.
When they stopped testing seasonal, and I guess now pretty much everyone, some people thought it meant people would start coming to work high. The thing is people are going to do what they want regardless if there are preemployment tests or not. Another thing to remember is if you have a accident on the clock your still going to get tested regardless, alcohol or drugs.
Another thing. Most of the hard drugs that do cause a risk are out of the system in a few days anyway. Meaning most drug tests won't catch them.