You will notice the Seinfeld plots don’t revolve around the celebrity and that makes a difference. George buys Jon Voights car. But that is not the plot, it could be anyone’s car (except Liam Neeson :)).
So the writers aren’t really appropriating anything. If you can change the name of the person randomly and it won’t effect the plot you’re not using them.
Also you can’t copyright facts, so you can use them. OJ was charged with murder and was found not guilty. That’s a fact. So as long as I stick 100% to the facts, I am on safe water.
Part of the misunderstanding when topics like these come up is the fact people are so litigious. You can be wiped out in a lawsuit, even though you’re right. If you make a movie and pay for it, then spend the next ten in court fighting a lawsuit, by the time you win, your movie could have no meaning.
So it’s easier just to get the permission of a celebrity or famous person first. It eliminates any hassles before they arise.
Name dropping is fun but it can backfire. For instance on Night Court, Jude Harry Stone would constantly talk about Mel Torme. OK that’s funny and they don’t need his permission. But producers are likely to get it because, not only do the writers have a right to free speech but so does Mr Torme.
Supposing he didn’t like it, he could go around on talk shows saying how much Night Court sucks and how stupid it is. He has a right to his opinion and opinion is strongly protected by the 1st Amendment.
So a lot of times waivers are put in simply to prevent any trouble from coming up in the first place