Why would Johnsonville as a group wish to continue poisoning Tablesville’s water supply if the Tablesville community makes it clear to them that they are being harmed by Johnsonville’s lack of adequate treatment?
Easy. They don't believe it. They think Tablesville is exaggerating. They think Tablesville is confusing what is causing the polluted water. They think that pollution isn't that bad. They think that their need to spend more time with their kids in their very short and mortal lives is worth more than Tablesville's need to reside on a very specific piece of land that Johnsonville can't even see the point in inhabiting. They don't care about Tablesville. Take your pick.
Johnsonville would likely be receiving mutual aid from Tablesville due to their close proximity, so it’d be really weird of them to willfully screw over their downstream neighbors whom they often exchange help or supplies with?
That presumes that the level of mutual aid is substantial and bidirectional. If Johnsonville is in a good position and largely helps, rather than is helped, while Tablesville is a barren little scrap of swamp, what need does Johnsonville have of Tablesville's good will?
It would make sense why Johnsonville would want to skimp on water treatment under a capitalist society, as perhaps there are some corporations that don’t want to deal with treating their waste water, so they lobby the local government to allow it. Profit motive can often overcome cooperative goodwill and empathy for others.
Bruh, people will put other lives at risk to end a job - not a capitalist job, but everything from volunteer work to self-improvement - a fucking hour early.
You don't need capitalism to provide a motive for overcoming goodwill and empathy.
But in an anarchist society where there is no profit motive? Not saying it’d be impossible (perhaps Johnsonville is weirdly anti-science for some reason and won’t listen to reason?), but it’d be a damn sight less likely than the same scenario under Capitalism.
You could make that argument, but that presumes that this is a binary choice between anarchism (in this distinctly non-enforcement sense rather than libertarian socialist sense) and anarcho-capitalism, and that's not the case.
A democratic socialist state has the obligation to enforce the laws made by common agreement upon all members of the polity, even those that disagree. Even a libertarian socialist polity has that same obligation, it just has more layers of decentralization which prolongs how long a problem must linger at low-level resolution before the central polity comes in.