They may have insulted him but he just didn't handle it right.
In his public letter posted on Facebook, he said: " So I wanted to ask you a couple of questions, and I wanted to do it publicly because I think it's important that people know what your answer is." Why didn't he wait for their response to a private inquiry? Maybe they would have agreed with him and offered him real money. He didn't know what they would do at that point, yet he went public. He was stewing for a fight.
In the letter he also admitted: "I'm not accusing you personally of being hypocrites, I don't know how involved you are in this process, but I'm letting you know it's happening and it's happening in your name." If he didn't know what the musicians knew, he should have waited for them to respond before going public. It appears he was looking for a fight or for the publicity. Also, by going public first, he put them in an embarrassing position to defend themselves. Maybe they are "garbage". But he still lost his moral high ground by attacking them first in such a public way.