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He lied to the stewards in order to gain a benefit, rather than just telling the truth. Why do you think this is?
As you rightly said Hamilton was in 3rd and let Trulli past - this was a mistake, but guess what? Tough. It was his mistake and he should have sucked it up and lived with his error.
Instead he and the team tried to recover the position by lying to the stewards about the circumstances of Trulli overtaking him - they trying it on and succeeded in getting Trulli a penalty when in actual fact Trulli had acted in good faith, honestly believing that Hamilton had an issue and was about to drop out of the race.
Then they were found out by the stewards to have cheated - and they have got up and admitted this themselves.
He may not have pushed Trulli off the track, but a cheat is a cheat and deserved to be treated as such.