Training to be an ATC on VATSIM. I live in the Boston area, so I'm training with VATUSA. In the US, METAR visibility data is the only exception to 'miles' = 'nautical miles' rule (that I've learned about/am aware of). All other use of 'miles' including speeds (knots) are in nautical miles.
It seems dangerous to have multiple standards for the same unit of measure, especially when something you want to underestimate is given in the smaller units (1NM = 1.15SM), meaning an error results in someone expecting greater visibility than they actually have (and possibly overestimating their ability to safely fly). I'm hopeful that there's a sensible reason for this, but I live in the US so I recognize it as hope as opposed to reasonable expectation.