They shouldn't and that's exactly why states are allowed to declare bankruptcy and why Puerto Rico should be allowed to declare bankruptcy. States in the EU probably should be able to also.
Due to these uncertainties, and undoubtedly also to pressure from the rest of the EU, Greece decided instead to renegotiate their debt with the Trojka.
Now that the debt has been turned from public to private, it might be easier to handle as a situation, but I doubt the EU is able to handle such a crisis gracefully.
Greece is stuck to a never-ending extend and pretend program which will effectively kill the Greek economy in the 21st century.
:-)