The simple answer to this is that it cost a lot to commision a piece of music from a talented musician but there are LOADS of bands out there knocking on the door of fame and stardom all looking for a way to break through to a bigger audience and they aren't going to charge much (if anything) to let them use there music when it could be their gateway...remember Babylon Zoo - Spaceman? never heard of before and never since all down to the power of an advert...It has happened in the US with Brit bands, T'Pau to name one, that couldn't give their stuff away and then along comes a Pepe Jeans advert and you have a Billboard chart topper.