Once cannabis officially becomes legal in the United States and the world, we can expect that law enforcement will treat it differently as well. We can expect that lawmakers will take a page from the book of alcohol enforcement meaning that there will be specific instances where the consumption of alcohol or in this case cannabis becomes illegal once more.
For example, smoking cannabis while driving remains illegal even if marijuana is legalized on the federal level. The difference in crime is that no longer the possession of the substance alone would constitute you breaking the law.