It is policy, and it is good to do, and everyone would like them to be dimmed, but it’s not a no-go issue, for one simple reason:

”Cabin Crew 10 mins to landing”
”Crap the system is broken. Hmmm I can’t make the lights go off”
”Hello Captain, yes it’s SCCM here. I’m afraid you can’t land, the lights are on and someone might die not being able to see in the dark”
”Oh yes I see. Ok. I’ll stay airborne until you work out how to fix it”
[A bit of flying around in circles]
[Bang/Crash/Wallop]
”Oh dear, we ran out of fuel, seems we’re in a field over Enfield”
”Oh look Captain, everyone’s dead”