Usually that's called a "monopoly" lol. TBH no label "supports it". Labels have their stuff on there b/c that's where the mainstream money is. If you're getting bigger and your stuff is crossing-over out of the strict dnb fans, the money is going to come in from itunes with casual fans hearing a tune on the radio and d/l your song to stick on their ipod/phone. For anybody who is a DJ or producer, or just simply a music connoisseur, I just don't get using itunes at all.
For labels, itunes gives you very little say in anything over your own material and horrible return rates. You have to do everything there way, or no way. You have little-to-no control over prices or other aspects. If it works for you, cool. I just don't like the limitations itunes brings for both labels and ppl using it as a personal computer player/music manager, nor do I like running a beast of a program just to play a few tunes on my computer.