Unless someone really screwed up, any logo you see is absolutely an advertisement (with the exception of live format stuff like the news). They have to get permission to use brands, just like clearance for songs.
Are you sure? What law would you breaking by having someone drink a specific brand of soda on screen in a short movie? Trademark? What if it's topical?
What if there is a scene that involves a car, such as driving a car? Should movies ideally "make" their own car, or is there a way to use an existing car without being accused of product placement?
Copyright violation for reproducing their logo without permission. One could probably make a case that it's fair use most of the time, but why go to the trouble when you can instead get them to pay you to have that logo on the screen?