Reference:
Cambodia Publicly Endorses China Position on South China Sea | Voice of America
Cambodia publicly (on March 25, 2015) said the South China Sea is not an ASEAN issue. I think Cambodia made the right decision for three reasons.
1. Pick the winning side. China is the undisputed military, economic, and technological heavyweight in Asia. Siding with the winner is a time-honored tradition.
2. Facts on the ground heavily favor China. China has a huge naval base on Hainan Island. China is building a ring of heavily fortified air and naval bases on reclaimed islands in the South China Sea. Since Chinese control of the South China Sea is inevitable, going along with the flow makes sense.
3. Looking into the future, everyone knows Chinese J-20 and J-31 stealth fighters will be coming to the South China Sea. With a proven and relentless double-digit annual increase in the Chinese military budget, siding with China is the only realistic choice.
In conclusion, Cambodia made a wise realpolitik decision. Bismarck would be proud.