So because the laws say the bowler doesn't have to attempt to bowl if he's going to mankad.
To play devils advocate: If the light is closing in, or it's starting to rain, could the bowler keep running up and mankad the batsman, even if he's not out. In doing so he kills enough time that the game has to be called off?
It appears that could be within the laws although outside the optional spirit of the game?
The reason why I think mankad is immoral: The bowler hasn't beaten the batsman through skill or ability.
If the batsman had set off blindly like the WC Courtney Walsh incident, when he gave a warning instead of breaking the stumps, then yes the batsman is trying to gain an advantage and with that comes a risk.
That is not what the Zimbabwe batsman was trying in this case.