snip: In regards to EU sanctions, 1 vote out of 28 can kill the deal. That's a lot of leverage, especially when 27 on the other side want something from you. What are they willing to offer in return?
In contrast, when it comes to bailouts, Greece is outvoted by a huge margin, perhaps 18-1 within the Eurozone block. In this case, Greece desperately wants something from the other 18 instead of the other 27 wanting something from Greece.
The only way to negotiate when it's 18-1 against you (and you are the one who needs something) is to have some leverage. If Syriza teamed up with To Potami and agreed to sanctions, Tsipras may as well put all his cards on the table saying "here, take the ones you like".