Because targeting infrastructure like that is a war crime.
Mind you so is starting a war of aggression, perfidy (attacking while under a flag of truce), and a bunch of other things the US has done, so I'm not sure why they're trying to cover this one up.
No clue. You’d have to look for the pattern of things the U.S. almost certainly hit and later denied.
The Al Jazeera article suggests hitting water infrastructure is always a war crime, and then references a non-binding memo, which hints at the fact that the claim is bullshit, something cursory searching confirms. That’s probably not it since both the U.S. and Iran have proudly owned up to and threatened war crimes throughout this stupid war.