you are grossly mis-interpreting the actual will & sentiment of the greek people. Tsipras is not going to present his parliament or his people with a deal that he knows cannot be passed. That would be laughable and career and political suicide. There will be 0 probability of ramming through a deal that the public doesnt support. Since the two sides are vastly apart (on tax, and pensions) its a non-starter at this point, Tsipras knows that he cannot ever gain enough momentum or votes.
By calling a referendum he will allow the greek people to decide its fate, not politicians, bankers, or the elite. The entirety of the greek people will decide what is acceptable. The original bailout never went to the people. It went straight into the central bankers to provide the greek financial system with liquidity. That is where a lot of this mis-trust comes from. The greek people are immovable on pensions, absolutely immovable because of this. They are very jaded by the original bailout, which did not provide enough actual economic stimulus but rather liquidity into the financial banking system.
This is simply a final end game for greek in this era.
The guys a communist and he's been playing this game for 5 months, so he can leave the Euro and blame the economic crisis on the EU and join Russia and China.