It is not possible to place buttons on blocks occupied by mobs or players

You are not able to place buttons on blocks occupied by mobs or players. I expected to be able to place them.
Steps to Reproduce:
1 - Get a button.
2 - Stay on top of a block.
3 - Aim to the block below you.
4 - Try to place the button.
5 - Ready. You will not be able to place the button.