Not withstanding the attendant moral issue of "why is grass worse than booze?" the bottom line is one of the law.
You are permitted to work with a very small amount of alcohol in your bloodstream and alcohol is a legal drug.
You are permitted to smoke yourself to death with legal cigarettes, even though they have a very unpleasant effect upon your body's ability to extract and transport oxygen.
You are not allowed to have ANY marijuana in your bloodstream because it is an illegal drug.
Now, fair or not that is the law full stop. Those people who think that it's wrong that tobaco is legal and marijuana not may have a point - however, the answer is that if both these drugs were discovered for the first time this week it would probably be the case that neither would be made legal, not that marijuana would be likely to join tobaco on the "acceptable list"!
Drinking excess alcohol is legal - it may not be smart, but it's legal - unless you want to drive, fly, perform neurosurgery etc. You will find plenty of rules regarding alcohol use in SOPs and other aviation related documents. These tell you how much you can have and when to stop. If you can find similar advice in an SOP - telling you how much marijuana you can smoke and how long before flight you must stop - I would be interested to see it.
Remember, marijuana lingers in the system a lot longer than alcohol and an increasing weight of medical evidence suggests long term psychological harm can result from the use of even small amounts.
Think also of this - beer, wine and spirits are carefully assessed and graded so that you know exactly how much alcohol is in there. Therefore, with a little common sense, you can work out how much you've had and when to stop. I have yet to find a governing body for marijuana production which assesses and grades it for strength.
The bottom line is that it's not "do gooders" trying to stop people having fun, or recreational dope smokers enjoying something that they believe should be legal. The bottom line is that it's about the law, about safety and about keeping your sanity, your licence and your job.