Because if you accidentally click it and waste an action youre going to be annoyed. You can click anywhere, it doesnt have to be on you.
Possibly, but you can always cancel the choice in the same way as for the other bonus actions. If they want to warn the player that they have selected Dash then maybe the 'extra' move line selection should be in a different colour or dotted or something. At present it is just mildly irritating and annoys me every time I need to double-select it.
