Andy, all professional wheel manufacturers paint wheels, they are not powdercoated.
Powdercoating is cheaper, it cost me £10 a wheel in the past for acid dipping, bead blasting and powdercoating - simply because it is an industrial process and quick and easy to do.
Go to any 'powdercoaters' and 99% of the time you will see their main business is powdercoating and galving gates, railings, sheet metal etc... functional coatings, whereas when you have wheels painted by a 'bodyshop' you know their main emphasis is the final finish. It is not for function, but for pure aesthetics. End of the day, you can get a paint finish to look perfect, powdercoating can not get to that level..
Most people like to fork out for the extra, of having a painted finish. If its good enough for bodywork, its good enough for wheels.