Yes, the pixelating tech is cheap and easy. Mark a section of an object and the software can follow it with relative ease. It is hardly "CGI". And there can be reasons other than advertising for blocking a logo. In the Food Network case, maybe it was a slogan that seemed innocuous to the production crew, but which the lawyers decided might offend somebody, so they blocked it out. There's currently a commercial for a new disposable diaper from Huggies that looks like denim shorts. The tag line is "the best you'll look pooping your pants". Some stations are running an alternate version of the ad with a different tagline because they think some of their viewers would object. Different strokes for different folks, I guess.
Or maybe Sara was wearing a t-shirt from a brand that was a sponsor when they shot they episode, but which had since dropped out.
Finally, you're assuming they were pixelating a logo of some sort. Maybe it was just cold in the studio that day and the S&P folks got nervous. 
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wouldn't a random appearance on a TV show be covered by fair-use laws?
Not necessarily.
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Section 107 [of Title 17, United States Code - the Title that defines copyright law] contains a list of the various purposes for which the reproduction of a particular work may be considered fair, such as criticism, comment, news reporting, teaching, scholarship, and research.
News reporting, yeah, other uses, no.
Also don't forget that things like the Nike "swoosh" aren't copyrighted, they're trademarked - and trademark law is a whole different animal.
Finally it isn't always the trademark/copyright holder who has the issue. Sometimes the station or network just decides it doesn't feel like providing free publicity for brand "x" when it is in the business of charging for advertising. If brand "x" wants to get its logo in front of WXYZ's viewers, it can pay like everybody else.
Regards,
Joe