Let me backtrack and play "devil's advocate" against some of my previous arguments. Perhaps there is a meaningful difference between true "folk" music and all other genres, and it's this:

Real folk music is not conscious of itself as a potentially commercial commodity, but is produced for the enjoyment of a close local community, family and friends. Lots of stuff done in the past certainly met this criterion (and, for many of us, our own musical activity fits this defintion as well). But, as far as songs are concerned, almost any contemporary composition that we all know (from recordings widely heard and recognized on both sides of the Atlantic) falls short of this strict definition of "folk."

It's almost impossible, in the present age of communication, to be as purely naive as the anonymous authors of the old trad songs, but (probably precisely because of the pervasiveness of commerically recorded musical "product") there is an impulse and a movement to preserve music that sounds and feels like the communal music of the past.

For those who allow a looser definition of folk music, it seems inconsistent to accept some 20th/21st century compositions while turning one's nose up at others ~ assuming, of course, that all these pieces are being performed at an approprite volume level with acoustic (or mostly-acoustic) instruments.

In other words, if a venue's policy to to restrict performances to a truly traditional repertoire, then yes, it makes sense to rule out Beatles songs. But on the other hand, there is absolutely no reason (except prejudice and hypocrisy) to accept, say, Bert Jansch's "Anji" while excluding the Beatles' "Blackbird."