pot kettle
Strange as it may seem, when UMG are found to be selling music to which they have no rights their attitude is "provide proof of exact amount of your loss and we'll start negotiating, otherwise forget it" (oh, and no we wont make the relevant accounting records available to you); and in the meantime we may or may not stop selling such music because its a pain to organise what with all our subsidiaries and the world being such a big place, and anyway we have better things to do.
Hopefully iTunes and similar organisations have a legal arrangement with suppliers like UMG that covers them from getting caught in the crossfire when they supply "illegal" material in good faith.