Drug Testing by Employers

With the impending decriminalization of marijuana here in Canada, does that mean that marijuana will be removed from drug testing by employers?

When being screened for employment purposes, they don't test you for alcohol consumption unless you do something wrong like operating machinery while intoxicated.

With marijuana being legalized what difference would it make to an employer if the tested employee smoked pot 36 hours earlier? For that matter, what business is it of an employer as long as the employee didn't show up to work high?

By Cannabis Training University (Own work) [CC BY-SA 3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons