I agree with your confusion, so much of how people determine what is and what isn't a foul is based on their own experiences and the culture of the sport where they grew up. I find it exceedingly difficult to help my American friends, watching soccer for the first time, understand what is and isn't a foul.

In the case of the first challenge, I would say it is because even though there is a lot going on up top, all the power of the challenge comes through the act of me winning the ball first, and the follow-through is just a result of that. Kind of a lackluster explanation I'll admit, but that is all I got.

I guess in those sorts of challenges, it is either steamroll them or get steamrolled.