In such a raid the police would not ordinarily arrest ALL of the customers present. Unless Japanese law is quite different from most other First World countries, it is not illegal to be present in a location where some violation of the law is taking place. A customer only gets arrested if he/she is *found* to be provably doing something illegal...which might not even be related to the purpose of the raid (i.e., people get arrested for outstanding warrants, carrying drugs or weapons, visa violations etc).
-Ww