States aren’t required to enforce federal law, and the federal government can’t force states to help enforce federal law, as it’s unconstitutional.[1] If the feds want to enforce the marijuana ban and the state doesn’t volunteer to help, the feds would have to use e.g. the FBI or DEA to enforce the law. The feds don’t have the capacity to do that nationally, though, so it mostly goes unenforced, except maybe for large busts where it’s worth it for the feds to put the effort in.

[1] http://en.wikipedia.org/wiki/Commandeering#In_the_United_Sta...