I was wondering that too. The first few times I heard someone say "President Clinton" after he was out of office, I thought it was just a wishful slip up. But now they do it all the time.
That's the correct form of address for all former presidents.
From what I understand, it is proper to address all former presidents as President. I don't necessarily agree with it, but there you go.
It's no big deal. Many military folks still are called General, Admirals etc after they retired. They never say former. That is stupid anyway. We only had 43 so they all deserve the honor of President, even President Clinton as much as he doesn't deserve it. You respect the office and not the person.