Society broadly agrees, enough that it's illegal in the US to stop someone just for "unusual behavior." You have to have an actual concrete reason to suspect someone of a crime. Not that police always follow the law on this.
Only in specific edge cases and definitions, which I’m guessing you don’t know. And calling it ‘illegal’ is a stretch in 95% of them. Generally worst case any evidence gathered would just be inadmissible.
After all, even if not a legal stop/detention, that doesn’t mean they committed a crime by doing it.
But tell me, do you think any of these officers would have struggled to come up with probable cause to detain the driver of a giant banana car on a public roadway? Or any other ‘suspicious’ or ‘weird’ vehicle?
Because I can think of at least 3 California vehicle codes off the top of my head that would likely apply, including CVC 26708, 24008.5, and 5201. And I’m not a cop.
And all you need is an articulable and reasonable suspicion to detain.
Stopping someone to chat (aka they can leave without penalty) is a much lower bar, though I doubt they did that.
And it didn’t start there in Germany, either banana cars or death camps.