While we cannot deny the ability of the coach, it is my understanding that experience is something that stands out on top when compared with aspects of sportsmanship. While it is not alright to play against what the coach has already called for, it is not entirely wrong to sometimes apply the experience especially when a player is out in the field and under immense pressure.
LeBron is an experienced player and during the play he might be able to analyse the situation a bit better than his coach sometimes and its perfectly fine to substitute his own play provided he has valid reasons for doing so. He has the veto power and he's got every right to use it.
In these situations nobody but the players on the court knows the best way to get out of these situations. I am not saying players should not follow their coaches directions,but sometimes in a sticky situation the coaches play recommendation may not be the best way out and the player may have a better play to get out of the situation.
A player should never veto a call by the coach. Only ego would make someone like LeBron feel he should do so. Out of respect he should just mind the play. LeBron is not a coach and he is not an owner. The owner gets to decide overall what happens, and he has placed the coach under him.