The relative minor might explain why it works over can't buy me love, but I think he was asking in general why minor notes work over major chords. Technically they don't, but in blues they do. Same thing with singing, it's ten times better to sing flat than sing sharp, a lot of times it gives it a blues feel, and easier fixed.

But this should explain what I was trying to articulate before
http://www.fretjam.com/minor-pentatonic-over-major-chords.html