There's a permit/fee required for commercial photography, not specifically for LF cameras, although laypeople tend to conflate the two. That's hardly out of the ordinary in our own countries. If you're polite, respectful, and firm, it's unlikely you'd have to pay anything as a hobbyist.

Not that I imagine many people would to begin with, but be wary of trusting police officers. You can buy a full police uniform at the markets in Phnom Penh and Siam Reap, or at least you could when I was there in '08.