It's complicated（法律はないそうです）

//www.talesofasia.com/

Q: I'm pressed for time, can I see Angkor as a day trip from Bangkok?

A: Yes, you can, but it's complicated. Bangkok Airways will not sell a same-day return ticket from Bangkok to Siem Reap, so you have to play a bit of a game with them. If you purchase a round-trip ticket, either set the return as open or book it for any day, say a week later. Then when you're ready to leave, turn up at the airport and tell them you have an emergency and must fly back to Bangkok. You also can purchase a one-way ticket to Siem Reap and then when you get to Siem Reap purchase another one-way ticket back to Bangkok. Contrary to what Bangkok Airways might tell you THERE IS NO IMMIGRATION LAW that says visitors must spend one night in Cambodia. The land crossings are full of visa runners from Thailand whose stay in Cambodia can be counted in minutes.

途中略、

While I wouldn't recommend doing Angkor as a day-trip, I realize some people have legitimate reasons for doing so, and one day is better than nothing. It may be a rushed and exhausting day, but yes, you can do Angkor as a day trip from Bangkok and people have done it.

決まりはないということは、不文律・・・ですか。
嘘も方便ということでしょうね。

これこそ、Good Luckの世界でしょう。