Well, even without free-floating currencies, countries like Greece could just lower the wages, which would in turn reduce the price-level of Greek products, making them more competitive.
In a truly free market situation the bond market likely would have stopped buying Greek bonds because of the fear of currency devaluation long before their structural problems became overwhelming. Sometimes though the bond markets make a bad bet and currency devaluation becomes necessary. That's the free market. There's risk in buying bonds. However, in the EU the bonds have an implicit guarantee from the ECB, Germany, and France.
There's a reason why this kind of thing might be true, because bond holders might hope for intervention, as indeed happened. But I doubt it in this case: (i) the Euro treaty forbade intervention, and (ii) the CDS markets, which one would think would be very sensitive to risk of default, were not worried until shortly before the bond markets proper were.
But note that the Euro is for the purposes of this situation equivalent to the gold standard. Fixing exchange rates means trading the risk of currency depreciation for an increased risk of default, as we seem to be assuming the bonds are issued in the local currency.
The issue here seems to have been ignorance on the part of bind investors to the reality of the Greek situation.