There's certainly should be no prohibition against discharging an employee for doing something "bad", whether it's misconduct or poor performance - or even for being under the influence of alcohol or other drug while on duty. There are ways to prove those things, if necessary, other than testing the employee for the presence of any substance in his or her blood/breath/urine/hair. Some states have explicit legal requirements that must be met before an employer may test for the presence of any drug in the employee. For example, an employe may be required to have a written drug and alcohol policy that is distributed to all affected employees, as well as training of the affected employees and the supervisors who'd potentially be accusing any one of them of having a drug in his/her system. There also may be other legal requirements, such as the opportunity for voluntary "rehabilitation". As for certain employees, such as Commercial Drivers License [CDL] employees and employees working in the nuclear powered electric generating facilities, federal laws and regulations apply.
The criteria for establishing numerical limits are based, sometimes, and in part, on scientific testing of the amount of detectable substance in the blood [by testing blood, breath, urine, or hair] as against measurable behavioral deficiencies. Some of the numerical limits, however, are based upon public policy - for example, CDL drivers, or automobile licensees under the age of twenty-one, may be in violation of a law or regulation if ANY amount of the prohibited substance is detected in them while operating a motor vehicle on a public highway. Many drugs, including alcohol, have a certain relatively brief period during which they're metabolized or excreted. However, the psychotropic substance in marijuana is stored in fat and hair, and it may be detected at various times well after being ingested either by digestion or inhalation. A weekend pot smoker of a joint or two will not have the same amount of the psychotropic substance stored as will an every day heavy user. If an employee stops smoking pot, it is possible to detect the substance as much as month or more after the last use, depending on the level of use - and the drug level in hair basically stays there until the hair falls out or is removed.
Some skiers may have noted exceptionally mellow lifties at some areas [not where I've ever instructed], and in a seasonal market in some locations, the level of tolerance for such "mellowness" may obviate any chance that such person will be fired [assuming that the job is getting done and the guests aren't making major complaints]. Other areas are expected to be less tolerant of readily observable "mellowness".
Regardless of political or personal philosophy, the subject of drugs and the law - and especially drugs and employment - are complex and not always subject to facile resolution, even though DUI laws have no doubt saved many lives and their violation has cost many more. Laws vary from state to state, and attitudes can vary legitimately among cultures, religions, and families. And we don't know all of what there may be to know about drugs. We're imperfect, as is our knowledge, and we all want to do the best we can with the imperfect knowledge we have.