I've heard this from multiple people explaining stuff to me -- they were all Americans.

Where are you from? If it's from continental Europe, I wonder if that's the difference. We get our laws from English Common Law instead of civil law (except Louisiana).

Can we get a brit in here to see what their conception of the law is?

How courts interpret legislation and precedent differs a lot between common and civil law jurisdictions, but that's got nothing to do with how policing operates. Police everywhere can choose not to take action unless you're talking about a zero-tolerance society with Stasi-like levels of monitoring.

"Selecting policing" is just "discretion", that particular phrase is possibly US-centric but the concept itself isn't.

I would imagine how likely courts are to convict has to impact that discretion for any police force that's judged based on some sort of metric.

Or is the US the only place that really does that?

I'm sure every police force takes that into account along with other factors, there are always laws that become outdated and are arrested for less, prosecuted less and convicted of less over time. And in common law jurisdictions precedent can impact the scope of existing law and change how police will enforce it.

I'm not American.

I misread as “the” people and was confused. Sad.