Umm, if Madonna was going to be good for iTMS, it would be because she sold a buttload of music through them. If she can't move song sales, then she doesn't deserve more money that the one hit wonder who likewise can't move massive song sales. If she CAN move song sales, then she gets a significantly larger paycheck by the wonders of multiplication.
I make a lot of money from the company for which I work. It's not because I've been here long; it's because I provide a distinct value to that company. How? My experience with the products is certainly a factor there, as is my general industry experience. At the end of the day, I contribute to more sales of the product, which is logarithmically related to the amount of pay I get over Joe College Graduate stepping through the door.
"Seniority" by itself is not an asset. That having been said, there's a lot of Madonna's catalog that I'd snatch up on a per-song basis. Bad taste, of course, but I revel in it. I just can't stand buying an album of hers for two songs and gritting my teeth through the other ten that have no emotional or otherwise significance to me. As it is, she loses out on my sales, and iTMS gets the same budget allocation regardless, and I'm probably better off in discovering a new artist I'd never heard of than indulging my reminisces anyways