Even the criminal justice system has no qualms about "grounding" (taking into custody) people before they are found guilty in a court of law.

Question: is it realistic to expect cops to know exactly what law someone is breaking before they decide to arrest them? Like I get that in an ideal world this would be the case, but humans only have so much memory. Is it unreasonable if a cop arrests someone because it seems the person is doing something that warrants it, but then later fails to find an actual crime they can charge them with? i.e. Would we be better in world where cops only arrested those whom they could immediately charge with a crime?

Cops arrest people for all sorts of reasons. They don't have to have proof of a crime; suspicion is enough. They don't immediately have to charge them with anything, and they can first question you. If they decide not to charge you, they can keep you in custody for a couple of hours at most.

Then again, in Guantanamo Bay there are people held in custody for many years now without any charges, so these rules can be bent pretty far in the right circumstances.

Planes aren't criminals or even citizens, though. But to translate this to the 737 Max situation, when something seems to be wrong it seems entirely reasonable to ground planes in order to investigate the situation. But there should be a limit to that grounding, and a permanent ban requires some actual proof.

Of course keep arresting people for no good reason, people lose their trust in them.

In practice, cops in a position to arrest people on their beat are generally arresting a series of people for the same thing over and over again, and eventually learn the details of what will satisfy a charge.

Mileage will vary based on geography and beat though.

And... I'd admit we saw some charging documents we immediately dismissed from new officers, roughly like 'this guy seemed shady and was near a crime.' We'd lay into them for that though, they'd eventually seem to figure it out.

You'd have to ask a lawyer if this would fall under probably cause - i.e. a police officer is allowed to arrest someone if they have a "reasonable" suspicion that a crime was committed, and then the prosecutors decide whether to bring charges. It's not clear to me how a court would view an officer being mistaken about the law in specific ways.

This depends upon where you are doesn't it? In the UK the grounds for arrest are "reasonable suspicion" in the US isn't it "probable cause"? I think that "reasonable suspicion" is similar to 10% certainty and "probable cause" 25% (c/f with "beyond reasonable doubt" which should be 95% certainty)...

I am not a lawyer. If anyone is in doubt I suggest speaking to a lawyer!

"Probable cause" is defined using the word "reasonable" in lots of sources. The root of any UK/US difference is more likely related to drift in precedent over the last couple of centuries than to any precise difference in the meaning of words.

I don’t think the parent comment is about arresting. But holding someone without bail until a trial has concluded where by it is deemed that the person is a possible risk to society by being released until trial. (I think)

So if I don't like a black person I can just arrest them because he seems shifty?

Effectively yes, you can do that as a cop in most justice systems if you can dress up the "seems shifty" in a bit more concrete terms.

But if there is nothing to back it up, the guy will be realeased very soon, and if you do it repeatedly, you'll eventually get problems (and rather quickly and seriously if you lied and it can be proven).

A little bizarre to see "them" followed by "he"...

I guess the question here is what about him makes him seem shifty to you. One would at least hope his skin color would not be factoring into that and that your judgment would have been the same for anybody else exhibiting the same behavior.

Great point. Criminal Justice is about erring on the side of caution of imprisoning people wrongfully. Its why there's a difference between arrest and imprisonment.