Think the whole "for law enforcment only" thing got started back in the 1970's with the old Treasury Department +p+ 110gr. JHP .38special. About the same time, Illinois State Police had adopted the 9mm (S&W 39's at first) and wanted a load with a specific ballistics...the "Q" loading, but I forgot how it got that designation. Could consider that the first +P 9mm.
Idea here is that these loads were above industry pressure, so some kind of distinction had to be made. the makers evidently had no intention of making this ammo for commercail sales at first, but as time passed and guns didn't turn into handgrenades, the concept of +p and +p+ as a commercial venture was too tempting (besdies, they had some R&D expenses).
Not illegal to sell it to civialians as far as I know; have bought boxes marked "law enforcement only" at several local gun shops over the years.