Presidential debates (after the primaries) are handled through a common outlet and monitored. In the past, clearly democratic news media have indeed hosted Presidential debates. The forum, questions, etc are all approved beforehand. I think everyone is confusing the Fox debate with those. There are other non-official debates (but the forum is the same) prior to the primaries which the Fox debate falls into that category. Everything is agreed to in advance and there really is not a debate per se (which always disspoints me). Questions are posed by the mediator and each answers with an opportunity for rebuttal. More like little speeches on each question. In this case, it was to be hosted not only by Fox News, but by the CBC as well which is sad. The dem candidates are boycotting Fox News.

http://www.debates.org/

2004 debates:
http://www.debates.org/pages/his_2004.html

These are the applicants for 2008:
http://www.debates.org/pages/news_070402.html

It is sad that twice the dems have refused to participate in a debate because it was sponsored by Fox News. The debate was also sponsored by the Congressional Black Caucus Institute. There are typically a few of these prior to the primaries. In the past, debates have been hosted by CBS a clearly democratic media.