Here's how you knew FAR Part 6.3 doesn't apply to contractors - DCMA accepts "Customer Direction" as a valid source justification.
FAR Part 6 implements the Competition in Contracting Act on the federal procurement process. Here's the gist of CICA - the government has extraordinary power in the acquisition processes it initiates. Without public laws dictating competition, and the implementation of strict documentation requirements when competition is not used, that extraordinary power is too ripe for abuse.