Separation of Church and state violation the atheists will not mention
There's no violation of any "separation of church and state" in this case -- because the "Catholic Church" in the U.S. isn't even a church anymore. There's nothing religious about the organization at all.
The Church was originally centered in Rome because Rome was the seat of governing authority in the Roman Empire -- and the Church's mission was to convert the Empire.
In the U.S., the USCC and the NCCB aren't located in Washington D.C. because they have any interest in converting the nation's leaders. They're located in Washington because they're an arm of the government.