How long have you had the plant? Are the purple roots mostly older or younger?
My guess is that the plant has been treated with something, some form of fungicide or fertilizer or some other chemical shortly before you bought it which dyed any roots which were on the plant at that point (+/- depending on whether the medium retained any dye, or if the colour built up over several applications etc) and that the new roots that have grown in are green and normal since the plant is no longer getting whatever it was that was making the roots purple.
I remember reading something somewhere about roots being dyed for some reason. I forget where I read it or why they were doing it but this reminded me of that.
Just my guess.