They typically have no idea how any of this stuff works and just assume that the purpose of marking something is spam is to prevent them from seeing any more of it, personally, in the future. It doesn't occur to them that their preference thus exerts a small influence over the experience of potentially millions of other people.
In the decades past, when preferences weren't so tightly linked to each other among otherwise unaffiliated users, the simple definition of spam as " stuff I'm not interested in seeing in my inbox" was completely sufficient to inform a user's decisions about using the spam button. But today that definition is something closer to "stuff I'm not interested in seeing and that I am fairly certain few if any other people are interested in seeing, either."