They did that quite often because the programs the networks aired were on behalf of sponsors who were guaranteed their programs would air with little or no interruption in contracts between the sponsors and the networks.
Most of the special news broadcasts the radio networks aired on December 7, 1941 were in time slots where sponsored programs were not airing.
In some cases sponsors agreed to cut back or cancel their programs.
I assumed it was because of sponsor commitments, but it just sounded somehow odd.