It's just like employment discrimination. In an at-will-employment jurisdiction, my boss is free to arbitrarily fire me, but cannot specifically fire me for e.g. my race.

(How do you tell the difference? Well, as in this case, usually people leave some sort of evidence, and the legal system has a process for demanding evidence and evaluating it. It's not perfect, but it's certainly not unenforceable.)

You’ve got it backwards. Instances where they do leave evidence are the exception, not the norm. Which is why the anti discrimination laws are somewhat toothless and the government tries to look for statistical evidence of discrimination.

Does the driving test requirement to receive a driver’s license mean that driver’s licenses are may-issue?

Nope, that's also shall-issue in most states. If you meet the statutory requirements (passing the test, age, etc), then they must issue the license.

But the test is necessarily subjective. The instructor can determine, for instance, whether the applicant drove recklessly during the exam.

Sort of. I kind of hate that reckless driving law since it is somewhat subjective. Each state should have case law about it and allow you your due process rights to challenge it too. But yeah, I can see in the context of the test that it could be a problem. If you record the test, then you should be able to challenge it (sad it sometimes comes to that).

I'm in a similar situation, actually. I had a trooper lie in court and even admit to it in the complaint investigation, but the investigator (another trooper) said it must be a misunderstanding. I'm curious how intentionally lying to cover up your mistake (exculpatory evidence) is a misunderstanding as a civil rights violation under Brady v MD, but I also don't want to get 'suicided'.

It would be extremely difficult to define all driving rules objectively even if you had extremely high resolution data of the car and its surroundings. Approximately as difficult as programming a fully autonomous car, I reckon.

Not really. The driving laws except for recklessness are very well defined.

That’s not the case. There are tons of driving laws that are based essentially on a subjective idea of what is safe or reasonable. Laws about cargo being secured safely. Laws about the driver’s view not being unreasonably obstructed. Laws about the driver being distracted. Laws about exercising appropriate caution in response to traffic, weather and road conditions, the presence of pedestrians and other dangers and obstacles, etc. There are tons.

Not when you look at case law. Also some of that varies from state to state.

The part of the test administered by the state is objective, in all three states in which I've obtained a license.

The part which is subjective is deputized to instructors, and one can shop around for a willing one.

The moral hazard you're alluding to is completely absent.