Therefore, you'd have a reasonable case if you were fired for testing positive for THC in your system, as an employer only has grounds to dismiss if you are likely impaired at work. I'm sure there are many jurisdictions that handle this differently, but there is a fine line on Charter/Constitutional rights when it comes to D&A testing. Furthermore, if there is a chance you are "addicted" to a substance generally speaking the company has to find you help before they can just outright fire you. So the dependence argument comes back to bite us in the ass there. If we say it's not addictive, we can't argue that we shouldn't be fired because we have a "problem"...
Some employers have zero tolerance drug policies that are different from their alcohol and tobacco dependence policies. It all comes down to how the company's policy is written and what they're willing to tolerate from their employees. Just because a state relaxes something doesn't mean that an employer will relax it too.
While many employers have random drug testing policies in place (have you checked your HR policies lately?) they rarely do check except when one is hired. Depending on the employer, look for some of these "random" drug tests to actually be administered in the future.
Due to the rising cost of ammo, do not expect a warning shot! Sorry for the inconvenience.