I agree with you — BUT, I have conducted a fair amount of user tests where someone will not scroll. It's often an older demographic, but I've seen 30 year olds fall into it as well.

So it's not always a designer saying "I know better than you."

I do think that they're going about solving the problem the wrong way though. Your design should on its own encourage scrolling by making it obvious that additional content exists (avoiding so-called "false floors") — it can be harder to do this than unchanging your design and suggesting scrolljacking, which is why I think we see it so often.