"How does being able to pass an exam about points of historical interest have any relationship to whether or not you're going to treat people carefully, take care of them, not abandon them?" Brown asked.
The licensing process in Washington itself has a long history, driven primarily by the need to protect the safety of tour-group members. The law was first enacted more than a century ago, and newspaper stories from the time say tourists were being intimidated and harassed by unlicensed guides.
A federal judge last year found that the licensing requirement does not significantly infringe speech.