I was watching a movie the other day (“Welcome to the Dollhouse” - a very disturbing movie, BTW) and in it one of the characters is playing a Rickenbacker guitar, but the logo on the headstock was obviously covered up with black tape (or something like that). I’ve seen this type of “brand hiding” before. Many times it quite apparent the logo that’s being covered up: the Ford “blue oval” on the front of Ford cars is a good example. Why is this necessary? Do movie/tv producers have to pay when they show a product name? Seems like it would be free advertising.
They actually get paid to do that. I think that sometimes if the movie isn’t going to be huge, the companies don’t want their product in it.
I think the worst one for product placement is “Some Like It Hot”. Jack Lemon and Tony Curtis go into a “prop room” to find women’s clothes and bump into a bunch of things like a giant TurtleWax turtle and a giant Gilette razor and say lines like “Are you out of Blades?” or whatever the slogan was at the time. It was strange.
Also, some movies- esp indy ones- aren’t allowed to use products that the company has denied permission for. I saw this one movie called “Late Bloomers” about a lesbian couple, and the only product that they could have in it was hamburger helper or something like that. Any other product in the movie had a generic label. You also can see examples of blocking out potential advertisers on reality shows like the Real World, where logos are often blurred out. So yes, anytime you use a trademarked thing in a movie/tv show permission must be given- otherwise the company could sue because the product appeared in a movie that didn’t agree with the company philosophy.
It does work for other things. Danny Glover put the “End Aparthied” bumper sticker on the fridge in the house in “Leatal Weapon” as a statement. YOu’ll notice it stayed there. 
There’s an interesting story behind product “blocking” and Grease. The linked article is followed by some interesting letters about product placement or the lack thereof.
In Repo Man, everything was in the “generic” containers that were popular at the time: A white container with a blue band and black lettering. An actor would say, “Do you want some food?” and then go to the fridge where he’d pull out a container that simply said “FOOD”. Hilarious!