If anyone is a culprit it is the Commission on Presidential Debates who decided the criteria for [link|http://www.debates.org/pages/candsel.html|candidate selection.] Picking 15% of the poll vote as a minimum, I'll grant you, was not aa arbitrary choice. I would agree that a lower (but > 1) percentage would be better. You don't want a zoo either.
For Lehrer to have declined to be the moderator would be to become a news maker and not a news reporter. He would be making a political statement and raising a furor which would destroy the need to be impartial. What do you expect from the press or an umpire?