I don't see anything wrong with the letter; if you want to be paid for use of your software, that's reasonable. Gates basically expressed that he disagreed with the philosophy of the club. Nothing wrong with that.
What's happening with Jguitar however is that a big tech player that exists because people pay for their software (iOS) setting a precedent where they're actively siphoning content outside the garden and feeding it back to users in their ecosystem, with no attribution or licensing deal with the creator. That's wrong, in the exact same way that homebrew Hackintosh machines built for sale are wrong.
What's happening with Jguitar however is that a big tech player that exists because people pay for their software (iOS) setting a precedent where they're actively siphoning content outside the garden and feeding it back to users in their ecosystem, with no attribution or licensing deal with the creator. That's wrong, in the exact same way that homebrew Hackintosh machines built for sale are wrong.