He signed bills based on that referendum that put pot into play. He could have just as easily said it is against federal law and let it die.
You're forgetting Eric Holder and DOJ are not on board with that plan. They have an official nonenforcement policy provided the use and sale is withing specific guidelines.
The legislature did the responsible thing to enact a regulatory regime. Otherwise, there would have been chaos and lawsuits out the wazoo trying figure out what is allowed and what isn't.