It's his actual job - to represent and follow the opinion of the people. It's called democracy. And if the opinion of the majority is wrong, then, in this case, so be it.
He's not a representative, he's an administrator. Representing the people is the job of parliament, his job is to lead. He cannot just switch sides whenever it suits him.
We have all of these safeguards in place to ensure our leaders neither go off the rails completely, nor simply follow the popular opinion du jour with the predictable disastrous consequences.
If we would do that in the rest of Europe, aid to Greece would have stopped a long time ago.
Well, it should have stopped a long time ago... Or to be more accurate, financial help should never happen in the first place. But that's another long story.
To be fair his mandate on election was to reduce the impact of austerity. He has pushed for that but now the choice is austerity cuts or exit the Euro, neither of which he has an elected mandate for. Many other leaders would make the decision regardless but he had put it to the Greek people because ultimately it will be them that have to live with the decision and he knows that most don't want to leave he EU. So now they have to decide what is more important EU membership or no more austerity cuts.
However in reality it is no decision because without EU membership there is no money anyway so austerity is happening to Greece one way or another.