This excellent piece by Tony Barber makes the very valid point that having a NATO and EU member state essentially fail would leave an unacceptable power-vacuum in South East Europe. Given Greece's neighbors and the willingness with which they would extend their influence, Europe cannot and will not simply walk away.
Maybe Alexis Tsipras knows exactly what he's doing but he is playing a much more dangerous game than a €7.2bn loan would suggest.
It has a history of conflict and confrontation with Turkey. It has border disputes and other disagreements with Albania that are a latent source of regional instability. Finally, Greek diplomacy can, to a limited degree, play a Russia and a China card. Leaving Greece to turn into a “black hole” of south-eastern Europe is not an option, because leaving south-eastern Europe to its own devices is not an option.