It's certainly intentional behaviour:  all the defaultdict does is provide a __missing__ method.  And as explained in

http://docs.python.org/library/stdtypes.html#mapping-types-dict

"No other operations or methods invoke __missing__()."

So it looks to me as though this issue should be either a doc issue, or a feature request for 3.3.  Reclassifying as a doc issue.