It is based in part on advertising theory that advertising to promote your competitors may not, in fact, sway your audience, but will, in fact, give you advertising revenue from these competitors. Of course, Fox News and CNN, indeed, hope that MSNBC's viewers will watch them---that's why they're advertising in the first place.
This principle usually works on mailing lists--why, for instance, you might buy something at Gap, but end up with advertisements for Abercrombie & Fitch in your mailbox even if you've never shopped there before. I have never seen this done on television. MSNBC must really be strapped for cash.
Melon