I think the difference is one of outlook. In the abscence of other evidence, do you presume that a fielder is lying, or do you presume he is telling the truth? Obviously, the fielder wants his side to win, so its not exactly an unbiased opinion. So can you take the word of the fielder every time?
I think you have to, because cameras can't be everywhere, and in the end it comes down to trust.
However, it is unreasonable to accuse Dhoni of poor sportsmanship as he was not sure whether he was out or not, and you can't expect batsman to walk on every appeal. On the other hand, should Lara have been this adamant about Dhoni being out, whenI'm sure there's been quite a few times when he KNOWS he's out and yet has refused to walk?
Its a tough question, but I think they were both looking out for the team and I can't blame either of them. But I was still sad to see Lara acting as angry as he did - even if he really felt that anger. It's important to note that Lara didn't accept Dhoni's decision, but in the end Dravid accepted the fielder's decision and declared the innings.