Apropos of the Greek and Puerto Rico situations ...
Suppose I come to you for a loan, and when you ask about my income source I tell you that I am a mugger, embezzler and extortionist by trade, and that I intend to pay you back with revenues from future muggings, embezzlement schemes and extortion rackets. Would you loan me money? I don't think so. At least I hope not.
The only difference between that situation and "investing in government securities" is that in the latter case you're dealing with a criminal gang rather than an individual criminal.
If you loan money to politicians based on their promises to pay you back by stealing more from their victims in the future, you deserve to lose your money, and I hope you do. I don't owe you squat.