A 28 gauge handgun should be illegal under the National Firearms Act, without special registration and tax. Of course, that's a new law, and perhaps the word hasn't gotten around to everyone. It was only passed in 1934.
I have never actually read the law, but have been told that it does not say a shotgun handgun is illegal. Instead it states a smooth-bore handgun is illegal, so if it has rifling it is legal.
There have been .410/45 Colt handguns around for a long time. The Judge is not the first. Most are single shots except for the BFR.
Edit: Info on National Firearms Act of 1934
http://en.wikipedia.org/wiki/National_Firearms_Act#Categories_of_firearms_regulated Here are two extracts from your link:
Short-Barreled Shotguns
Short-Barreled Shotguns are defined as shotguns which have a barrel length less than 18", or an overall length of less than 26". As with short-barreled rifles, this term refers to any firearm made from a shotgun as defined.
Any Other Weapons
........A shotgun with barrel length less than 18 inches originally manufactured as a handgun without a shoulder stock is also an AOW..........
Thus makes the Judge 28 a NFA weapon. This judge does not fire an alternative cartridge in .550 and only fires shells.
Notice I said it's a NFA or (title II) weapon; not an illegal weapon. You can legally own a title II weapon with the paying of the $200 Tax and $5 transfer tax.