In the past I have seen people wearing USA (and sometimes Kenyan) singlets during rinky dink 5k's and turkey trots. Some people seem to hold the view that they are sacred uniforms and wearing one without earning it is disrespectful to the sport and reduces the value of earning it. (You can buy them on ebay or through USATF).
I think it's cool to wear one and show your support for US distance running. It's no different than a kid wearing a Michael Jordan dream team jersey.