The derivatives that underwrote the debt made it all ok though. Same thing happened with the mortgage bubble. It's an easy way to turn a bunch of garbage into a bunch of AAA prime credit stuff. When the whole thing collapses, all the bankers get 100 cents on the dollar courtesy of central bank printing and sticking it to the taxpayer.
The key difference here is that the banks have already been bailed out (when international institutions like the IMF and ECB took over the loans), and now those institutions are demanding that instead of the taxpayer picking up the bill, the Greek pensioner do so. Shame.