Thank you. I hadn't checked what they're issued with.
So then I'm wondering why he didn't demount the flashlight. I guess he was worried that he might accidentally fire the gun into his foot while doing so. He's obviously a little bit prone to that kind of thing. Safer to leave it on the rail, I guess.
You asked "you can ... but why would you?". You answered yourself.
Because he wasn't in a situation where he was going to need to shoot anyone but he decided to proceed as if he was. And accidents happen, as he demonstrated.
You can say this is SOP, but that's worse, isn't it?