SQL Server 2014 - Add Node inherits wrong configuration
It appears to be a bug/feature. It was discussed previously on SQL Server Central http://www.sqlservercentral.com/Forums/Topic1578488-2799-1.aspx where Ed Vassie said:


  Put it another way... On an ADDNODE install you cannot specify the
  location for the SQL binaries, they always go to the system drive.


Ed Vassie is a reliable source as the original developer of FineBuild, a tool specifically for rolling out repeatable installs of SQL Server including in clustering mode. He lives and breathes this part of SQL Server.