Once again, it comes down the same dichtomy found in Jazz, Trad and Progressive. The idea folk is American I find hilarious, but occasionally offensive: America has its heritage in Europe, mostly in the UK. Every area has its own specialities, sure, but do cowboy songs belong in Folk or C&W?
That Mumford are called Folk is a matter of costume, not custom. That BD is called folk was a mistake of the 1960s, thankfully rectified in the 1990s: how glad I was on returning to the UK after many years overseas to discover the midAtlantic adenoidal with a guitar has nearly disappeared!
Another of the old questions is one of origins. We sing from who we are, and that includes who our family are. It means nobody in the UK is likely to sing cowboy balleds, for example: perhaps gymkhana ones...
At the same time, some traditions are specific, such that particular songs in some areas (Essex, for example) can "belong" to a particular singer to the point nobody else sings them.