"Let me ask a question: 'Why force the government to buy cars that were made by thousands of unknown people?'."
So you know everyone who built your car? I don't... in fact it was probably made by thousands of unknown people... or at least hundreds. Sure I know which company made a profit from it, but I don't know any people. If I paid for an open source solution, and support for that solution, I WOULD know who had acceptted responsibility for that software.
Frankly, if a NATO general released the missile guidance software back to the community it would be acceptable to me. It might even work in the favour of NATO... "OOOH, look, their missiles are accurate to within 1 metre. Ours aren't that accurate..." The only people who would be able to make use of that software would require the hardware as well.
Sure Ford could release the blue prints to their cars... it wouldn't do most people any good whatsoever. Remember, you can already go to an auto parts store and buy all of the parts. Knowing how to stick them together isn't too difficult.
While I wouldn't argue for enforced use of only open source software I find your analogies to be somewhat poor.
More important to my mind is that the data file formats should all be open standards. That way YOU could write some software to manipulate them, I could write some software that worked differently, THEN their is real choice, rather than the current "well, all of our current documents are in this proprietary format, converting them to that proprietary format is a real pain" pseudo choice.