Political parties are private institutions. In fact, political parties have often asserted the private right of private association to prevent unwanted influence on party activities. Yet, prior to 2012, every state in the union, except Washington and Louisiana, had partisan based primaries. Therefore, in almost every state, nonpartisan voters pay for the nominating process for political parties in which independent candidates cannot even participate. In many cases, the ability of independent voter to participate is limited or barred completely dependent on the form of the primary. 

Read more at Independent Voter Network