It's a daft thing to get in a tizzy over, true. I'd just always assumed it was a short form of Joseph as it is for, ooooh, a good majority of folks called Joe.
I always felt that Fargo called Dredd "Joseph" in the film (and later in Origins) because he'd given him the name. The rest of the world can call me "Mike" until the cows come home, it always sounded a bit odd from my mum and dad.