It's very simple, inside the eurozone you can transfer your euro's from dangerous Spanish/Greek/Portuguese bonds and put them into safe German bonds. Even if the euro were to collapse, the German bonds would be redenominated in the new German currency, chances are it would immediately begin to rise vs other currencies.

So you don't have to exit the eurozone area, you just transfer your money from the debtor states to the creditor states. That doesn't make the situation any better particularly it just drives up bond yields in the debtor countries and makes their situation even worse.

Essentially a decision has been made to save the currency and sacrifice the debtor countries which use it. This is just another example.