Technically, federal law still preempts in this particular circumstance. The issue here is of discretionary prosecution. All prosecutors (or the executive branch if you will) enjoy general discretion when seeking to enforce laws. It is almost ministerial in nature. This is so to enable the executive branch to efficiently manage resources when deciding to enforce certain statutes. Also, some laws, while still on the books, are no longer considered a social menace and therefore not enforced (but could be if necessary or desired).