Actual laws are up to congress etc which frankly don’t understand the intricacies because it’s not their job. So it’s common for agencies to be given authority to oversee something without a law explicitly defining specific level of salt in drinking water etc. Regulators therefore don’t make laws only clarifying where boundaries exist (safe levels > X ppm).

Deference for unintentional ambiguity seems unrelated, but in the real world people want to know where the lines are so they can respond accordingly. Not knowing where the limits are gets expensive for anyone not trying to push boundaries.

Lawsuits meanwhile are horrifically inefficient in terms of time. What exactly are people supposed to do while waiting for a lawsuit to finish? For some things sticking with existing guidelines works but nobody wants to make major investments when the underlying rules are about to change. Clarity is far more valuable than generally perceived and that’s what’s being destroyed here because the courts even decades to make the meanings of laws clear.

This decision is therefore directly and significantly harmful to the US economy.

The word "can" is doing a lot of heavy lifting in that statement.

In theory, I entirely agree - regulation should not be decided by agencies, but by lawmakers. In practice, this is so painfully far from reality. Do you really think congress has the ability to pass meaningful legislation on complex issues? Do you think that lifelong politicians can do a better job than civil servants who have spent their entire lives studying this particular issue?