We have had lots of debates which excluded candidates. The League of Women Voters is famous for not inviting third party candidates, and our local public broadcasting station has excluded third party candidates as well. Ralph Nader was removed from the audience from a debate he was excluded from. While it's legal to exclude candidates from privately sponsored debates, it does not allow people to hear from all the candidates. I don't think it's right.
I remember a CNN Republican presidential debate hosted by Anderson Cooper where he didn't ask Huckabee or Paul very many questions. Huckabee and Paul even asked for more questions, but their requests were ignored. CNN also deleted/did not print comments on their live chat pertaining to their bias.