The color restriction does seem strange, all right. Bolder, "signature" colors would do a lot for Alfa's on-street recognition in the US. People mightn't recognise a Stelvio straight away in traffic, but they'd spot that metallic ochre color. Ironically, this is a strategy that has worked well for Dodge - you can get a Challenger in a range of colors that none of the other makers would ever dream of offering, and it makes them really stand out in a sea of black, white and silver.
Technically speaking, it looks like it's a special paint process: possibly a three-layer with tinted clear-coat, like the old "8C Competizione" red offered on Giulietta. Alfa previously used BASF (Glasurit) as the supplier for that paint, so it may be their product again. I don't think that would be a reason for not bringing it to the USA, though - any decent paint-shop will carry these paints, especially with the much bigger market for custom-painting in the USA