Why can't I vote on iTunes Radio songs?
According the Apple support doc for iTunes Radio:


  There may be some DJ-curated stations that you may not be able to
  modify. You can't like or play more tracks like this while listening
  to some DJ-curated stations. You can however, start new stations based
  on the current track or artist of a featured station or add tracks to
  your iTunes Wishlist from the Star (✭) menu.