This priest is just making his views known.
He has ordered a ban on communion for John Kerry.
No he hasn't. This "memo" was published in an Italian newspaper. It has not been released publicly by the Vatican; we don't even know for certain if Ratzinger is the author of it.
If he chose to speak publicly and release this document publicly as head of the Congregation for the Faith, then it might influence the American bishops.
As it is, the bishops have voted to allow individual bishops to make their own decisions about refusal of the Eucharist.
Ratzinger has had extraordianry power for many years.
He's the power behind the papacy.