HMX doesn't say anything about ANY band not releasing content. We haven't heard them try to explain the lack of Led Zeppelin, Pink Floyd, Tool, or any of those bands. Or for that matter, why we haven't seen any DLC from other previous Rock Band artists like Garbage or Rolling Stones for ages. It's just not good business to come out and say "Yeah, Muse is asking for too much money" or whatever the reason is.
And please don't take this the wrong way, but it's not the requesters' fault that HMX keeps absolutely silent about why no Muse is released (although they probably have a good reason).