Little upside or revenue streams
Were Apple to spin off the iTMS it would pose challenges for any media company - music, video, TV shows, etc. - that wished to sell content through the service, especially if Disney were to own a stake (however large) in iTMS... They'd have to ask themselves, "What kind of preferential pricing & revenue is Disney making off our competitor-status product?"
Also, any media company, especially the major "greedy" music labels who invested in it could try to pressure iTMS & Apple to raise prices, tier the prices, limit the bit-rate, push for certain content to be subscriber-only, etc. And it would do nothing to move iTMS closer to being a "border-free", truly-global music & media bazaar.
The latter is where I believe iTMS is headed, where iTMS and, more importantly, its global customers, are no longer restricted to what's available in only their home countries. I see the day coming, maybe by next MWSF, where we can preview & purchase content from any of the various international iTMSs.
As it stands right now, Apple - and, if IPO'd, the iTMS - sees very little actual profit from iTMS' sales. Most of it goes to the record labels and/or movie/TV studios. Apple makes its money from iPod-related sales, not off the media itself.