Malware is software that is explicitly designed to disrupt, damage, or gain unauthorized access to a machine.

You are covering the unauthorized access but disrupting/damaging is absolutely possible using plain old HTML and JS.

Privacy advocates argue that it's not only possible but many trackers are guilty of exactly that.

So the browser is in fact blocking malware.

... And yes, if you think about it, that definition does apply to ads as well. Really says something doesn't it :)