Code analysis by marcono1234 can be found in this comment.

Marker-ArmorStands should not be able to be interacted with, but despite that, pistons can push them.
Screenshots for visualization/proof attached.

Steps to reproduce:

• Place a piston, sticky or regular, pointing upwards
• summon an ArmorStand on top of that piston, with:

/summon ArmorStand ~ ~ ~ {Marker:1b,NoGravity:1}

The piston can push the Marker-ArmorStand upwards.

This affects all directions:

• For horizontal directions make sure you summon it above the ground, standing on the ground and summoning it causes it to be underneath the piston head.
• Pushing downwards only works if the hitbox of the ArmorStand (which is for Marker:1b ArmorStands about the height of the base plate) it at the position of the piston head.

I can understand that it is "works as intended" that NoGravity:1-ArmorStands can be pushed by pistons, as they are entities, but it should not be the case for Marker:1-ArmorStands.

From a mapmaker's perspective, it'd be a desirable outcome if Marker:1-ArmorStands could not be interacted with at all, aside from commands.
So a pushing with pistons shouldn't be possible at all.