No one said it was. But some energy sources produce far less pollution than others, and are far safer. Also, everything I'm saying about regulating and taxing pollution from PoW applies to every single energy-intensive industry; PoW just happens to be the particular industry at hand.
> If we want to alleviate this, I suspect that we'll have to consider what the underlying social factors are that are driving the demand for such a network.
This is literally what regulation and taxation could address. If you tax cryptocurrency transactions based on how much pollution their miners create, the problem could fix itself -- miners would then be incentivized to use only clean energy to mine (and prove that they are doing so), since that's the way to stay competitive. Also, because literally every transaction is a public record, a government could (a) require KYC for you to trade crypto -- up to requiring you to register your address(es) -- and (b) send you a tax bill each year for the pollution caused by each block that included one of your transactions. Exchanges operating under its jurisdiction would only let you cash out once you paid that tax.
In addition, suitable regulation could require that no more than X% of a utility's electricity can go towards mining (e.g. only during off-peak hours; only if its excess; etc.).