It's not like I'm going out of my way to prevent advertising. Of course I don't cover a logo of my car, but I also don't drive cars on races or other events where I am publicly presenting something. If I were a famous car driver or even running mechanic shop, I would make sure not to show any, eg. tire manufacturer poster unless I have clear financial interest to do so.

But, if I'm presenting something computer related to an audience interesting in computer related stuff, it's perfectly reasonable not to show any unpaid advertising related to computers. I don't want that anyone in the audience get an impression that I recommend any specific laptop brand.