Why was Gulliver confined by the custom officer? On what grounds?

Gulliver travels to Luggnagg, posing as a Dutchman. He says, "I thought it necessary to disguise my Country, and call my self an Hollander, because my Intentions were for Japan, and I knew the Dutch were the only Europeans permitted to enter into that Kingdom." His true identity is discovered, however, and Gulliver is held by the Custom Officer.