Is it about the use of a private API or just the violation of the Human Interface guidelines? Everything I've read has said it's just about the guidelines, not

Is it about the use of a private API or just the violation of the Human Interface guidelines? Everything I've read has said it's just about the guidelines, not because an API needed changing. For me it's all about the timing. They should have relaxed the guidelines and/or policy about reusing the volume buttons for other purposes (not just for use as a camera shutter, presumably) at some point before they released an implementation of the exact same idea as a third party.