How can you grow an economy with workers high on weed? We do random screening for drugs as required by our insurance coverage. Without random testing or insurance coverage we have no company, and would probably move the plant to Singapore or somewhere that pot is illegal. There are severe financial consequences for out of control drug use.
Having drugs be legal just means government can't stop you from buying it. Companies are still free to test for it and it can clearly be restricted for things like driving, just as alcohol is now. The point is to take the choice from government.
If my sister sits at home and smokes pot in her house at the end of the day and is driving nowhere, why do you care?