It seems to me that the basic Counter class should be left as-is, and if there are specialized methods used for statistics (such as normalize) it should go into a subclass in the statistics module.

The statistics module already uses Counter internally to calculate the mode.

It makes some sense to me for statistics to have a FrequencyTable (and CumulativeFrequencyTable?) class built on top of Counter. I don't think it makes sense to overload the collections.Counter type with these sorts of specialised methods.