Summary:
Some people go to an party for which the expected bribe for event "license" the had not been paid, so they get arrested.
Omitting the bribe is not following Khmer culture.

Later news reports claim the police are after further bribes for their release.

That seems quite plausible, since they could satisfy basic law enforcement requirements by jailing them for a short period e.g. 48 hours & then deporting them. Thus clearing the space for the next offenders.

Of course, there is no money doing it that way.