Excellent point. As I understand it, Darpa does not preclude recipients of funding from patenting their work, nor do they insist on publication. I believe the govt gets some sort of free lunch on the patent side -- they can use anything they want with some sort of IP override (if someone knows more about this please explain).

The way the govt avoids vendor lock-in (in principle anyway) is by farming out the money to multiple vendors, especially as the progress moves towards real practical deployment. They did that with autonomous vehicles, and they're doing it again with legged robotics (http://www.darpa.mil/our_work/tto/programs/darpa_robotics_ch...)

However in this one area (legged locomotion) it seems to me that BD is far, far ahead of anyone else, unless that work is taking place in complete secrecy.