I don't see how whether they cannot withdraw because the government blocked withdrawals or because the bank has no money to give away matters. The riots are caused by inability to withdraw money in any case.
No, they weren't. To quote the Wikipedia[1] article on the riots: "The uprising was a predominantly middle-class uprising against the government of President Fernando de la Rúa, who had failed to contain the economic crisis that was going through its third year of recession".
1. http://en.wikipedia.org/wiki/December_2001_riots_%28Argentin...