Powershell: Why do I need to escape a double-dash parameter in $args?

Here's a very simple example script:

function fun () { "AAA $args ZZZ" }
fun a b c
fun a - b
fun a -- b
fun a '--' b
fun a --- b

The result I get is:

AAA a b c ZZZ
AAA a - b ZZZ
AAA a b ZZZ
AAA a -- ZZZ
AAA a --- b ZZZ

I apparently need to escape the double dash. Why?

I'm writing a powershell wrapper for a suite of scripts and some of those scripts assign a specific meaning to "--". How can I pass it through unmodified?