Firstly they never said that they would turn him to the US. They said that they would turn him to a third country. And only if irrefutable evidence of his guilt were to be provided. And they said that the only would turn Bin Laden, not any other AlQaeda members.
Algo this happened after they US started the bombings. Before the US attacked the Talibans refused to even talk about it. And they would probably just tried to make time to start a truce until Bin Laden left the country.
So it was actually a fake proposal and only after their previous refusal started the bombings on Afghanistan.