About the Why, you have to ask the Creator.
I have no idea. Besides that dynamic collision polygons the size of a dynamic tilemap is a No Go.
Pin works, as long as you do not try to rotate the tilemap. So you could pin it to a Sprite (as a face).
But i guess, that is not helping you, you intend to animate the collision polygon.
If that is the case, you gonna lock up the object inside solids, using a sprite or a tilemap, dont matter.