It's because Apple products are instantly recognizable but the logo gets covered up in media for legal reasons. A big BBC sticker on the back of a macbook just makes it even more obvious that it's a macbook because it draws the eye to it and often the light bleeds through the thin stickers anyway

Which is kind of the point.

Apple paid a shit ton of money over the years to get their devices into enough tv shows and movies and even students at universities to do that. They want to be the default so that you have that "Oh, wow. Gwart has a Dell?" moment. And they want the writers to know that so that... frigging Gwart has a Dell.

As for covering it up? The prop department doesn't care about Apple's image, they just do what the production team tells them. And the production team doesn't care about Apple's image, they just want to make Apple pay for it.

As for in real life? I know a lot of people who put stickers over anything that is an LED that lights up when you use it. And I've personally always put a sticker or two on my work and personal laptops just so that I can identify it at a glance. You haven't lived until you have watched people have to check the lock screen to figure out which laptop is which.