I don't think anyone has touched on how tipping is really deeply integrated into Cambodian culture. When a driver/guide brings a good paying customer to a business..... the proprietor of said business often tips the driver/guide as a measure of good will to express his gratitude for having a good paying customer delivered to his door.

Your driver/guide has undoubtedly been tipped plenty on account of driving you around. Whether you noticed it or not.

Often this is quite innocent, fair play and part of the culture and has no more negative impact on you than if a western business offered you some bonus for signing up a friend. Sometimes it plays out in much more sinister commission racquets.

But that is the way "tipping" integrates as part of Cambodian culture. Trust me your driver/guide was tipped.