Some kids actually like listening to political sermons, jeremiads, etc., so for them "preachy" lyrics are a plus, not a minus. Generally speaking, Americans think highly of preaching as a form of art (religious folks often shop around in search of good sermons, for instance), so we shouldn't underestimate it. Besides, kids like preaching more than adults, in that they tend to be more righteous & self-righteous than older & sadder people, both for better and worse.
I think leftists can & should learn the art of public speaking from good preachers, especially African-American preachers.
As for the Rage against the Machine, its mix of hip-hop rhythm & metal guitar -- with punk attitudes -- has been a winning formula for many young white kids, probably because it synthesizes black & white working-class musical idioms & cultures. Lyrics may be of secondary importance, especially for youths.
Yoshie
P.S. I think Chomsky is popular because he is "preachy."