Like I said, the word "dialect" has a technical meaning: it's the narrowest phylogenetic category in a taxonomy of human speech (super-family/family/language/dialect). In that sense, sure, the distinction between languages and dialects can be arbitrary and subjective.
But most people aren't linguists, and as the wikipedia article notes, to most laypeople the term dialect is used as a pejorative, even if not altogether consciously. It connotes something that doesn't quite rise to the status of a language: the speech of illiterates, of tribal people. It's jibber-jabber. In everyday speech no one ever uses the term to refer to European languages. When's the last time you heard a non-linguist refer to (Castillian) Spanish as a dialect? Or Irish? Or Polish? It just doesn't sound right. On the other hand, talk of African dialects, or indigenous dialects, or tribal dialects, just rolls off the tongue.
FWIW I'm an Obama donor, and I don't think this reflects particularly poorly on him: right-thinking people make this unwittingly condescending mistake all the time. And there are plenty of smart, liberal people who actually do have it in their heads that Africans etc. don't speak "real" languages; trust me, I've had this conversation more often than I'd have liked. Having dedicated a couple of years of my life to learning one of these languages (and it was a difficult, complex, nuanced and beautiful language) it is now my thankless task to rid the world of this prejudice, one messageboard at a time.
8^)
