Maybe it is a matter of courtesy as George says. But I still feel surprised that an artist would somehow resent others selling CDs alongside theirs. As I said in a similar thread elsewhere, if your CD is any good people will buy it regardless of any 'competition'. I can't help thinking that anyone who would prefer to have exclusive sales rights is lacking confidence in the quality of their own product.
But what do I know, I don't have any CDs to sell anyway :(