A lot of states do not have laws for or against it. Texas is one of them for instance. The AMA's website is a good resource to find info on what's legal in what state and what states don't even address the issue of lane sharing.
Edit- They seem to have changed up their website a little bit. The used to put "Not Referenced" on there if there was no law on the books one way or another. Now they put "Not Authorized" But for states that have no law for or against they do not provide a link to the statute or code since it doesn't exist.