Greece isn't simply arguing that, morality being relative and all, they've decided not to pay back their debts.
They are demanding more money from their creditors. It seems like they have to: they're running a primary deficit; they're losing money even before you take debt service into account.
Whatever the moral dimension of paying back their debt is has nothing to do with the question of whether Germany (ie, the EC) or the IMF (to whom Greece just defaulted) or the ECB (which doesn't want to be in this particular lending game to begin with) is going to give them more money.
If Greece can't persuade the EC, IMF, or ECB to lend them more money, it's doesn't look like they can just walk away from this whole scenario. "Oh well! Debt is just politics and morality!" That might be true, but if they want to allow people to retire in their early 60s, or pay half the nominal tax rate, or not regressively tax the poor through punishing VAT increases, they're going to need to convince someone in the world of some kind of compatible morality, right? They require additional financing, don't they?