I'm not sure about the Grand Challenge, but a fair amount of work for DARPA involves the performers (companies/organizations who did the work) walking away from the program with full IP rights to the result with the ability to patent it or do whatever ... as long as the US government retains I think free license to at least that product/whatever and also maybe free license to anything produced by it? I think that might vary program by program.
DARPA is a "stimulating" organization. giving it patents would not fit into its mission.
