Well, seeing as how this part of Syria is run by al Qaeda- linked ISIS, what makes you think that NATO didn’t green light it?
Millions of ordinary people rely on that water supply and infrastructure who cannot easily relocate, while ISIS is a small band of highly mobile jihadists.
As a reason for stopping the water flow, 'fighting terrorism' is a complete nonstarter.
However as a provocation to prod Syria into a military conflict, denial of water to millions of Syrians would seem quite an effective tactic.