The middle man was getting a cut anyway. It was mainly to make China's purchases secrete.

  Primary dealers are not allowed to charge 
  customers money to bid on their behalf at Treasury    
  auctions, so China isn't saving money by cutting out 
  commission fees.

I'm not sure I can articulate it very well, but my general point regards the access given to core functionality in both military hardware and finance. This doesn't really comport with the US's relationship to China that is portrayed in the mainstream.