Fallout 3 offers you the giant robot screaming about how it will defeat communism fighting against the Enclave, meaning the PURE American values dealing with the "corrupt" US government as represented by the Enclave, so I can see why libertarians would prefer it.
FO3's writing is an abomination but Liberty Prime is one of the few great things in it. It's obviously exaggerated satire of the pure American philosophy, not an endorsement of it, and fits in well with the 50s parody of Fallout. It's one of the few highlights of that game, story wise.