Hi,
Once upon a time...
LEOs carried .38 Spls, and I have a box somewhere that originally contained Winchester .38 Spl "Law Enforcement Only" ammo. On the box is a notation that it's loaded to higher pressure levels than "standard" ammo. I'm gonna guess the box is old enough the ammo it contained probably would be called "+P" now that we have a SAAMI standard for same.
But today, w/ so many "higher powered" calibers available, and in wide use by LEOs, if that "Law Enforcement Only" endorsement's on a box of "common" caliber ammo, it's probably just as Slippingaway said... marketing, marketing, marketing.
And, since a bunch of it seems to show up at gun shows now and then, it doesn't seem like the stuff's any kind of "super special, keep it out of the public's hands" product: sounds more like a "Hey, we had a production overrun, send it to the outlet store!" thing to me.
Now, there IS stuff the public isn't supposed to be able to buy, but it's not likely we're gonna see that for sale online or at the shows.
Rick C