At the moment Arrows are having financial woes so it looks doubtful for next year. Also Cosworth having to supply engines to 3 teams sounds like a lot to handle. It's not just a matter of throwing the motor their way.
It's not a matter of wanting to swap Honda motors for Cosworth motors, but it's cause Honda want to concentrate on one team. Currently of the two teams, BAR is the more winning team so that's the way Honda want to go.