Generally, news agencies I think should have the power to control their own content, that is at the heart of the First Amendment. While I think it is generally up to the network, like MSNBC to determine what will appear on their programming, I generally don't think it is a good idea to exclude presidential candidates--but that is purely a business sense rather than a constitutional concern. MSNCB has planned a Democratic debate this evening and invited just Hillary Clinton, Barak Obama and John Edwards to the event. Democratic gadfly Dennis Kucinich was not invited and he threw a snit and sued to get onto the stage: And won!! Senior Clark County Judge Charles Thompson and threatened to cancel the event if Kucinich is not included. The network has vowed to appeal to the Nevada Supreme Court today in order to control their content.
I would say that MSNBC has a strong case for themselves, despite the presidential overtones. They do have a First Amendment right to control their news content. They are not using the airwaves (being a cable provider means that they don't have an FCC license and associated "public service" needs).
For Kucinich, this keeps his name in the papers but for not very good reasons.