I dont assume the cop in the video linked above wanted to pin something on the guy, he most likely really believed he was guilty. Thats the problem.
Cops are confronted with criminals every day and its their job to look for suspects. Its really easy to look like a nail if someone has a hammer. And once you are seen as a suspect, confirmation bias sets in. And with every word you say the risk of them finding something that looks to them like circumstantial evidence rises. Assume you said you are walking there every night, but once he specifies to yesterday you contradict yourself, because yesterday was an exception. Now add some dumb luck and you know the victim of what ever happened and you dont have an alibi. Now you returned to the crime scene, know the victim and lied to a cop.
Talking to a cop is simply reckless. They question suspects everyday and its likely your first time. You dont have to be guilty to contradict yourself in a stress situation, speech is a rather imperfect way to communicate.
And sure, they can suspect you either way, and there is nothing you can do about it except getting an attorney. Cops suspecting you of something isnt that big of an issue, there is a separation of power between executive and judiciary for a reason. Its the job of a cop to make a case against a suspect. Its the job of a judge and jury to evaluate if that actually happened.
I didn’t really get it until I came into a situation where a policeman was convinced I was up to something (not realising there was a breath testing station set up a few hundred metres up the road, I pulled over to make a phone call, and they came over convinced I was trying to evade them for some reason). Luckily it was pretty low stakes, and everything was in order, and the worst that was going to happen was maybe they might have pinged me if my tires had been a bit worn or something.
But it was extremely disturbing how throughly he looked for something to back up his incorrect suspicion, to try and find anything to build a case. He even said once he was done, as he told me to move on, that he was still suspicious! Having read so many stories of miscarriages of justice that have happened to innocent people, it was enough to show just how dangerous it can be to be completely innocent when a policeman suspects you of something. A guilty person knows they need to conceal, but somebody who is innocent often doesn’t know that the officer might have a very wrong hunch and might be looking for anything you say to make you a suspect.
It’s disturbing to wonder how it might have gone if, say, instead of them just checking for drunk drivers, something like a murder had happened nearby earlier in the evening...
I agree it's not a great equilibrium though. Police need to be able to get info from the community to do their jobs. But the problem is, who can trust them?
I don't know if the arguments in this video apply out of the US or not.