Well, I think it has a lot to do with marketing that places them as "Celtic." So what's wrong with putting whistle and fiddle in your songs? Is there some law that says nothing can have those particular instruments in without being strict folk? I mean, The Dave Matthews Band consistently features Boyd Tinsely's Cajum fiddling, but DMB is neither classed as "Cajun", nor do they pretend to be. In Roy Wood's 50's-esqu rave-up "Are You Ready To Rock" which is a kinda combo of Billy Haley & The Comets, and Big Band, he puts a pipe band in at the end, playing a pipe tune over the Fender telecasters and the double bass. I don't see anyone are here knocking ol' Roy boy for misrepresentation!