First of all, the president can't just wave his hands and legalize marijuana. There are three branches of government: judicial, legislative, and executive. The legislative branch is the branch responsible for debating whether or not to pass a bill. Even the president's veto can be turned null and void with a 2/3 congress vote. That's part of what's known as checks and balances - one branch's ability to check another's power, and balance it, in order to keep a specific branch from becoming too powerful. We're a faschist, capitalist, democratic republic, but we're not a monarchy (George Bush isn't KING.).

[sarcasm]I'm not positive on this, but I believe one of the prerequisites to becoming president is to know how your own government works.[/sarcasm]