You can fail a roadside for being sleep deprived and they have no way to prove that. You have to consider what circumstances one will come into contact with the police in the first place. People get pulled over while drunk because they are driving like they are drunk. Such is NOT the case with weed, or else it would be just as prevalent in the news as drunk driving stories.
Neil, I think you are being short sighted again. First, a hold harmless agreement will virtually eliminate the gunsmith from the equation. Second, how do you prove that he was intoxicated when he did the work? How do you prove that the work that was done even has anything to do with the function of the gun? The second one is easy, but the first one is impossible. Not to mention, that is what commercial general liability insurance is for. It is completely a non-issue. If you do bad work, you pay for it, it doesn't matter if you were drunk, or high, or sleepy, or forced, or in a hurry, or etc, etc, etc. It is all a non-issue.
One more example. If a mechanic is working on your car drunk and botches a brake job that causes you to crash, and it can be proven, then the commercial general liability is used. If the limits are exhausted, you sue the company and the individual. Life goes on. It makes absolutely NO difference if what caused the bad work was legal or not.