I think what is often called "folk" music, which is most accurately described as popular folk music is often left leaning, and that has to do with its origins.

In the late 50s/early 60s, bluegrass was beginning a wave of unprecedented popularity across college campuses for a variety of reasons other people can explain better than me. For as much as the northern college crowd liked bluegrass, though, some of its stickier points of conservatism and the conservatism of its singers spurned many of those young fans, who liked the music but disagreed from the often-fundamentalist nature of the songs.

While blues filled much of the interest from ethno-musicologists and the main popular tilt of college music, those who didn't turn to blues listened instead (or supplemented their listening) with their own versions of this bluegrass music -- "folk" music (aka 'popular' folk).*

So because this 'popular' folk music was created in reaction to conservatism and was progenated among left-leaning people, it has inherently always been a left-leaning idiom of music. You got groups who survived by-and-large on making left leaning records. One example I'll look at for a politified pop-folk group was The Mitchell Trio, whose last 'successful' album, Typical American Boys, featured at least 2/3 (or more) blatantly-leftist songs (some of their songs, in particular Yowzah, have not aged so well despite their good intentions)

Speaking generally about other types of folk music, their popularity among left-leaning audiences makes sense, too. The left tends to look more favorably upon the downtrodden and the more forgotten segments of society, and what's been more forgotten than America's ethnic past? From singing songs in foreign languages a la The Limeliters (who sang in Russian, Yiddish, Portuguese, Spanish and French) to dredging up nearly-forgotten protest songs like Pete Seeger, these aspects of American folk music appeal much more to the left than to blue-collar, right-leaning Americans, for various reasons.

There were, of course, some exceptions. The Lazy Aces recorded Dave Macon's ultra-Fundamentalist (with a capitol F to refer to the Christian group specifically this time) "The Bible's True" on The Young Fogies, or The Yetties' "I Touched Her On the Toe" (see here for a short discussion on that) but more often than not, what I've said above was generally the case.

So I guess in summary, I don't think folk is inherently political; it often features the music of/about the downtrodden, and often shares left-leaning views, but I don't think it *has* to be political despite their frequent associations.

* I should also mention part of the drive from bluegrass to folk was done in the interest of finding the roots of 'true' "American" music, but that wasn't always the main reason for many.