I grew up a Yankees fan and converted to rooting for the then hapless indians when I arrived in NE Ohio some years ago. (OK, my wife, a lifelong tribe fan would have probably thrown me out of the house for the Y-word as well). First of all, I though he was born and raised in Akron.... but he certainly has the right to root for whatever team he chooses.
HOWEVER... being a high-profile sports figure in Cleveland he should have just lain low on this issue.... and left the Yankees cap at home and kept his mouth shut... and if asked could have said something non-committal like "I grew up a Yankees fan, but I wish the Indians well."
Trivisonno makes a good point.... you wouldn't see Travis Hafner or Brady Quinn in the front row at th Q wearing a Pistons or Bulls jersey. When you become a highly-compensated athlete, part of that job is to be an ambassador for that city and by association, its other sports franchises.
To quote Michael Corleone from The Godfather: "Fredo, you're my older brother and I love you. But, don't take sides with anyone against the family again.... ever." In this case LeBron clearly disrespected the family.