A couple reasons:
1. To that point nobody had gone on live TV to announce their free agency decision.
2. Lebron didn't make an attempt to contact the Cavs to say he was leaving, he just went on ESPN to say he was dipping
3. Cleveland sports fans for the most part are paranoid....I believe anything bad can happen at any moment when it comes to sports. We haven't won shyt in 50 years. So when the best chance to win went out the window, people overreacted and took The Decision personal.
4. Good ol' fashioned 'cism. Some white folks think the purpose of athletes at the top of their sport is to do whatever falls in line with their ideology. "He plays for my team so this is what he should do." So in this case, Lebron making a choice and leaving the home team removes the only thing they liked about him, his ability to play basketball. Now he's just a ******. I bought his jerseys and his merchandise for myself and my family, and now he's spitting in my face. "I actually stood by this ungrateful black b*stard and come to find out he's got no integrity." No he shouldn't have gone on TV to do it, but ultimately its free agency and Lebron did what he thought was best...and it ended up getting him championships.