MPP’s Executive Director, Rob Kampia, wrote an article for the Huffington Post this week discrediting three common myths about the marijuana lobby most often spread by prohibitionists. It's important for all of us to keep our eye on the prize by agreeing that marijuana should be legal for people 21 and older; we'll put cartels and gangs out of business, and we'll have reasonable restrictions on advertising. None of this is new. Anyone who believes that alcohol should be legal should also agree that…