On the one hand, I understand their frustration. They are an organization, no matter what their size, dedicated to cataloging and honoring the life, death, and essence of a person - to not have control and/or access to all parts of that would feel like, for a very dedicated group, that they have failed to consider another aspect of this person they admire and seek to immortalize through their memory. Their goal isn't to "own" this person, but to give him life through all forms, long after his death, and they just want the best possible image out there for him, the most honest yet flattering image possible.
On the other hand, approaching you the way they had shows a complete lack of diplomacy and tact. Their e-mail should have instead focused on a shared respect for this dead musician and their wanting to work with you to make his WikiTree profile as beautiful as possible, rather than to claim ownership of it.
I'm sorry you went through this regardless, Eddie - you seem like the sort of guy who if they had approached you in a more civil, less entitled manner that you would have been ecstatic to work with them to rock out this musician's profile.
Regardless, I look forward to reading the ruling on this by the WikiTree Admin. I don't have any advice to offer, aside from keep your cool and calm when interacting with them. If they read my answer, I hope they gleam from it the sort of conduct they should be offering you instead.