One of the great mysteries of our time! Some say that the double negative suggests that they do need education, perhaps ironically. Alternatively it could be as you said, it just fits better rhythmically. However, also worth considering, a double negative doesn't necessarily mean the speaker intends a positive in English. For instance there are American movies where a character will say something like, "There ain't no goddamn way that..." or "I ain't never done such a thing!" and so the double negative serves to reinforce the refusal/denial etc. rather than reverse it, if you see what I mean!
I hope that helps!