"You could even be minding your own business (as both the bandleader and Woltz were) and if you had something Vito wanted, he would take it under threat of violence."

I don't think they were merely "minding their own business." Minding your own business would be a store owner who gangsters go to and demand protection money. The situation with the bandleader and Woltz was different. Particularly Woltz. In his conversation with Tom, Woltz made it clear that he was denying Johnny the role because of a personal vendetta and a desire to "run him out of the business," even though he actually believed Johnny was perfect for the role. While one could argue that Woltz had a right to cast whoever he wanted for whatever reason, Johnny, Tom and Vito still had huge reason to be pissed. Although one might seriously question whether killing an innocent horse was an acceptable response, Woltz was not some poor little innocent victim that had something terrible happen to him for no reason.