What does Obama have to do with it? He may be in favor of it,but he is not able to "allow" it. Any president (not just Obama) does not have the authority to just change the laws of our country like that. Not everything is his fault, public opinion has been changing on that issue long before him and we the people are the ones to decide not the president.
The Supreme Court is reviewing a couple cases right now. Having a patch work of states that approve of it and others that don't will cause problems but that's the direction the Feds are leaning - making it a State issue.