[ByteJockey]

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?

[spiralx]

I'm sure every police force takes that into account along with other factors, there are always laws that become outdated and are arrested for less, prosecuted less and convicted of less over time.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.