Really? Name one airline that does that
Lufthansa.
They were behind me in the crew channel at an Asian CX online airport about 3 weeks ago, half of them wearing civvies. When I asked what the situation was, they told me they were "dead-heading" (obviously their term for our PX).
They were quite shocked when I told them that although we can change to non-uniform onboard, we are required to go through the crew channel in uniform.