In that way, too, the Eurosystem is trying to avoid the moral hazard that could arise as a result of purchasing a disproportionately large amount of bonds issued by highly indebted governments. In addition to that, purchases are restricted to a maximum of 33% per country and bond series.
This limit was set so that the Eurosystem does not have to get into the awkward situation of having to prevent any restructuring of sovereign debt that may be deemed necessary. If the Eurosystem were to acquire too many bonds, it would have a blocking minority in any vote on a haircut.