It's important to recognise that that is YOUR definition.
And the truth is, both of the major parties are scared to have any different ideas introduced to the public. It muddies up their little game and may result in them having to answer some really tough questions. They sure don't want that.
If you really want to get torqued, go here!
http://usinfo.state.gov/dhr/democracy/elections/candidates_and_parties.html