The question is being over thought.
The reason for ammo makers not selling LE ammo to the public is simple. Repeat after me... "+P & +P+ is spec'ed above SAMMI specs".
It is all about liability. SAMMI specs account for a variety of weapons in a particular caliber. Some of the weapons might be new, some of the weapons might be antique. The old stuff is not as strong as the newer weapons. So they dumb up the spec to the lowest common denominator. So, if some old weapon or crappy pot metal weapon, or blow back weapon sets the spec for the lowest common dominator... that's the SAMMI spec for safety and liability reasons.
Some gun rattles to pieces from too hot ammo or grenades, the next thing to happen is a law suit now-a-days.
So, older calibers like 9mm, .38 special, .45ACP have specs to cover the lowest common denominator. However, newer weapons are much stronger and LE wanted the hottest ammo they would handle. SO... +P and +P+ ammo was made for LE in those calibers. The theory was that the agency accepted a portion of the liability by firing the hot ammo in stronger weapons, some of which the weapon makers said was OK with +P+ or +P.
Older 38 specials had different cylinders than a .357 by the same maker in earlier days. So, the newer .357 could take the hotter rounds, that might destroy a bored out 38 special cylinder. Now the steel used is the same... so it no longer matters and the new 38s can take the +P or +P+ rounds. The exception is most likely in the new light weight alloy cylinders.
Ever try to ask for or buy some 10mm, .40 S&W, .45 GAP (which actually is as produced near .45 ACP +P specs) or .357 SIG +P or +P+ ammo? Does not exist! Why? because the calibers are new and the ammo is already loaded to the capability of the weapon strength. Only older calibers have a +P or +P+ load. The new specs of the new caliber don't need to factor in old weapon strength. Their SAMMI specs are up to modern metals and locking systems.
Do you think it would be a good idea to fire +P .45ACP in an old Ballister Molina, or a COLT 1905 or 1907 .45ACP? You would at the least rattle the thing to parts. ...Or a 9mm Hi-Point carbine or KelTec SUB 2000 rattled loose by +P+ ammo. The Blowback action would be pounded to an early death.
It is not a LEO thing... it is a safety thing. The ammo makers can in theory control who get the +P & +P+ ammo to prevent it from being used in the wrong weapon. On the off the shelf open market, they have no control. People will put +P+ in some old WWII pistol. NOT A GOOD IDEA!
Another example... .450 Marlin caliber. Though it is not a belted magnum, it has a belted case, to prevent the ammo from fitting in weapons with similar chamber dimensions. It is a hot round... but technically NOT a belted magnum. BUT for safety reason a belted case was used to prevent it from being wrongly chambered in something other than a .450 Marlin chanber, as what it would have otherwise fit had much lower SAMMI specs, had it not been belted.
Safety and liability is the reason... not limiting the general public from having police stuff.