Obama's simply too conservative to sign a bill like this. He should, but he won't. The fact that marijuana is 100% safe isn't enough to sway the screaming, mindless Christians, and Obama needs at least some of their votes.
Why not? Osbama probably smokes doobies and blunts in the WH. He admits to having done so in the past. Oh, and no need to attack any particular religious group - it's over the top and unacceptable. There are plenty of agnostics who feel that maryjane has no place in legal society.