They are owned by different parent companies.
All the TV networks are owned by movie studios.
NBC Universal owns: NBC, Bravo, CNBC, MSNBC, Sci Fi Channel, ShopNBC, Telemundo, TRIO, USA Network, part of A&E, the History Channel, the Biography Channel, National Geographic International, the Sundance Channel, and TiVo.
Viacom owns: CBS, UPN, MTV, VH-1, Nickelodeon, TV Land, CMT, Spike TV, Comedy Central, Showtime, TMC, Sundance Channel, FLIX, and BET.
Disney owns: ABC, The Disney Channel, ABC Family, Toon Disney, ESPN, and SOAPnet. It partially owns Lifetime, E!, and A&E.
Time-Warner owns: WB, CNN, HBO, Cinemax, Turner Classic Movies, TBS Superstation, TNT, and Cartoon Network.
Fox Entertainment Group owns: Fox, Fox Sports Net, # Fox Sports Net (50% with Cablevision, owns many local FSN stations), Fox Movie Channel, Fox News Channel, Fuel channel, FX, National Geographic Channel (with the National Geographic Society), and the Speed Channel.
(credit: wikipedia)
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NBC doesn't own Tivo. Whoever set up this Wikipedia was smoking crack.