NASA mandates open source software for Ares avionics

Reportedly, NASA is requiring that the avionics software for the Ares I booster be 'open source' in accordance with this agreement. You can find links to several reference articles on the Rocketry Blog. Supposedly, this will allow wider competition for the Ares V. Well, it may make competition possible, but does not ensure it or even make it likely. Let's see, Boeing builds the Ares V but Company Z gets the avionics? No risk there, they have a pile of source code to get them started. Or someone else supplies the rocket and gets the old source code? Someone help me understand this.

P.S. - I'm all for open software and open standards, I just don't understand how open software on rocket A will help with rocket B. I'm sure the answer is out there...