It's quite possible that they had warned LeBatard and he decided to go through with it instead. I understand that ESPN has its right to its standards, but the whole billboard seemed more in jest than anything. I've read through Twitter and listened to his show, and LeBatard doesn't hold any ill will towards James. This was more of a slight jab at omission of thanks towards the Miami fanbase.
I'm not sure why James didn't thank the Miami fans. It's possible he saw the empty seats at 7:00 starts in the lower bowl and assumed Miami fans were frontrunners. However, I think myself and most Miami fans are very supportive. After the Decision, and especially the Dallas Finals disaster, we were the only ones who had James' back. He was disliked across the country, but Miami fans defended him. You look around Miami and see how many #6 jerseys there were. I don't think for a minute the omission was a mistake. That letter was carefully crafted, and the decision was clearly made before that Thursday night/Friday morning.
All James had to do was give a perfunctory thanks and Miami fans wouldn't have anything to complain about. I'm not going to go out and burn his jersey, but you can bet I'll actively root against the new Super Team that Dan Gilbert complained so much about in the past.