Fox News seems to have a real problem with cutting old footage into new stories. The liberal theory is that this is some sort of concerted conspiracy to imply that 80 million Americans turn out for every Tea Party or Sarah Palin appearance. The conservative line is that they were simply trying to punch up an otherwise dull segment, and/or somehow ran out of footage of the actual event. The latter story makes marginally more sense to me, but only because I cannot imagine that anyone at Fox News--indeed, anyone with a pulse--is stupid enough to imagine that a few shots of excess protesters are enough to marginally improve the chances of Republican victories in 2010 or 2012.
I'm not sure it really matters. Fox News was, I think, justifiably angry when the Obama administration declared that they were not a real news organization. But if you wanted to be treated like a real news organization, you have to abide by the rules that real news organizations follow. One of those rules is that you do not imply that images of one event actually come from an entirely different event. You don't do this for any reason.
It's entirely true that other news organizations have been caught in sleazy tactics, like the infamous habit of wiring cars to explode during auto safety stories. But they were righteously yelled at, and since the advent of the right-wing blogosphere, they seem to do a lot less of it. Now the left-wing blogosphere is fact-checking Fox with the same ferocity, and that's a good thing. We all have a vested interest in better news organizations.