My friend has worked in a number of different pre-school facilities throughout Boston and most of them had a carved out screen based activity (usually tablets) for at least part of the day. I'm sure the science is still out but in practice it seems ubiquitous.

That’s a perk for the teachers who can relax or get the next activity ready while the kids zone out, not done in the direct interests of the children.

Why is that? Is a coloring game on a tablet damaging? Is a coloring book not?

Personally I don't like anything with colors, since I see the world in black and white.

(I'm kidding of course, I agree with your point)

A coloring book teaches a billion times what a coloring app can teach. How materials feel, how to use haptic feedback to not break a crayon, how objects create sound as they touch, how different materials interact (eg pencil vs crayon), how friction changes based on force normal to the surface, how materials hardness affects abrasion and changes the shape of the crayon, how different items have different colors and how these can transfer between materials, etc...

Kids 2 to 4 should not be using screens. They have too much to learn.

The book and pages are tangible, foldable, the tablet is just a glass. With a book you think how ist could look, with a game you just can see it.