I agree, but a properly thrown slider simply must have some sidespin. Sure, some players get more than others; usually because of their arm slot.
The issue is one mostly of velocity, not orientation of spin. An overhand pitcher, who has a slider that is breaking straight down, is probably not getting the velocity on that pitch that he needs - in order for it to qualify as a slider.
If a pitcher who has a 90mph fastball and can find a way to throw an 87mph slider that breaks straight down - more power to him. That is very unusual not to mention not even typical, or even possible. My guess is that if it's breaking straight down, it'll probably, at best, be 80mph ... and that's very curveball-like, probably characterized as a "hard curve."
Most 90mph fastball pitchers who throw a curve throw it in the mid to high 70's. I've seen some in the high-60's. An 80mph curveball would be an extreme oddity.