A recent 1UP podcast with editor Shawn Elliot included an
interview with Epic Games founder Tim Sweeney, revealing that Microsoft is not letting them release free Gears of War maps. "We already released two (maps) and we have four more maps that we've built. We've been wanting to give them but actually Microsoft has been pushing back on us. They're trying to build this business model around selling additional content for games and that's a valid idea, but we would definitely like to release more stuff for free, and we haven't been able to do so yet."
UPDATE: According to Epic's Mark Rein, both parties have
made a compromise. The maps will be sold at first, then made free after a few months, much like the previous Halo 2 map pack.