A LEO can enforce any law in his jurisdiction. If the federal offense takes place within the venue, the LEO can make a arrest. The case has to be prosecuted in a federal court though. A law is a law, and it is the officers job to enforce laws. A city police officer can make a arrest on a county ordinance, state law, or federal law, it goes the same for a deputy, or state police.
Considering that most cases are tried in county courts usually state and local statutes are used for those offenses. It leaves the door open for federal prosecution should the local prosecution fail.