Well, I guess I don't really understand the Greek model of government. We don't have any equivalent to a national referendum in the US, but we do in California (where I live). It can actually be quite a pain in the ass at times because politicians can't just change their tune contrary to a pulic vote here.
If the same vote had been held in California, it would have created a legal requirement that the politicians not introduce new austerity measures. I guess I naively assumed that a similar mechanism existed in Greece. Instead it appears that Greek referendums are closer to Whitehouse.gov petitions in terms of political power.