This goes a bit beyond just cultural taboo.

A breathalyzer test can determine if you are under the influence of alcohol _at this moment_. No such test exists for weed. The only way to prove someone is not high is to do the usual urine test, which returns positive if you've smoked at all in the past ~month.

It's a liability thing. If a "weed breathalyzer" is invented and employers can prove that you are sober when reporting to work, I think smoking in the evening will become much less of an issue.