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 don’t care about Tablesville. Take your pick.
So that's assuming that Johnsville is naturally deeply uneducated, unwilling to listen to any evidence presented, won't test their own waste treatment output, or are majority sociopathic (lacking empathy for others), or a combination of all the above.
I could see perhaps a very insular and small religious fundamentalist town perhaps being capable of totally ignoring the problem, but any larger settlement tends to attract more education amongst the population. Our current system usually puts the sociopaths in leadership positions which can then override a community's wishes, but under an Anarchist system it would be highly unusual that the majority care so little about others to the point of not wanting to help whatsoever.
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.
In an Anarchist society, people would only really need to contribute about 2 to 3 months of work per year to have a functioning society that is able to provide everyone's basic needs for free. That would leave 10 to 9 months out of the year as completely free time for everyone to do with as they please, which would make it even more difficult to justify not spending a little extra time to treat your waste water properly for the sake not actively poisoning others.
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?
If they become so uncooperative and hostile to their neighbors, than they could receive negative perception or treatment from other federating communities near them, which would probably go a long way to encouraging them to just treat their waste water better.
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.
People are desperate to stop working an hour early because our current society gives them virtually no free time to enjoy life, to rest properly, or to not worry about needing to make ends meat just to survive and not become homeless. Most of their waking hours they are exploited with the majority of their effort going to the benefit of a few undeserving folk.
Would they be so desperate not to help if they were now afforded most of the year to themselves? I think many would find meaning in helping out in some of their spare time, since it is not longer exploitative or coerced.
You don’t need capitalism to provide a motive for overcoming goodwill and empathy.
It's doing the heavily lifting for most of society.
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.
A society of self governing communes could still federate with each other, and with that federation agree to some standards to become a part of that federation, such as adequate waste water treatment.