Yeah, you can't backoff from LeBron much... a running start for him is a big no no.
Lebron isn't Rondo. He doesn't have the same initial burst as Rondo. And is too big to get in the small seams. And remember there is always help waiting from our bigs. You backoff but enough where you close in when he's shooting.
LeBron has plenty of initial burst, and it's get exponential when you consider how big and powerful he is. So yeah, you back-off from him, but you certainly don't give him enough room for him to get some speed under him, particularly since he's quick, big, and powerful and can blow by SF's, such as Pierce, quite easily.
Help is important of course, but help is often late when LeBron is speeding through. Plus, you don't want to get them into foul trouble.