Its not though, the Americans with disabilities act doesn't cover it since its generally recognized as an illegal drug and the act only covers those with defined legal usages.

You might not realize but courts in California, Oregon, Colorado and Oklahoma have in fact ruled in court cases that you can be fired for off duty marijuaja use even when you have a legitimate serious medical reason to do so. I think the only state with laws on the books regarding marijuaba use that the court has stood up for is Maine, outside of Maine every legal state has allowed employers to fire its employees for marijuana use for varying reasons.

Here are some links to the laws and also court cases if I have the time to also link those

https://www.nolo.com/legal-encyclopedia/can-i-fired-because-i-medical-marijuana-disability.html

https://www.nolo.com/legal-encyclopedia/state-laws-on-off-duty-marijuana-use.html

Here's some links