I think a better wording would be that MLB "assigned" the DC region to the Orioles. MLB controls these assignments and per the franchise agreements between teams and MLB, teams only have exclusivity in the city/county that they are located in and the cities/counties that touch it. So, Baltimore located in Baltimore City would have exclusivity only over Baltimore and Anne Arundal Counties.
MLB assigns TV territories. Note that there was not similar disputes over Florida when the Marlins moved into Braves territory nor when Tampa moved into Marlins territory.
In this case the mlb caved and gave angelos the nats rights as part of a settlement to not challenge the expos move. MASN owns those rights, and it primarily owned by angelos
