Why repeal the don't ask don't tell policy?

It does not impact anything. Gays have served when they were in the closet for decades and they were heroes too just like any American. What is it now? And why is it that they wait until the serve and come back home to accuse them of violating this rule? It looks like they just want to give them a dishonorable discharge so they do not have to pay the promise money for the performance of their duty.