The man gave them his driver's license and registration, as required by law.
He was not required to give an accounting of his business to the police officer. The subsequent actions of the police in response to a citizen serf asserting his rights under the Constitution getting uppity with his betters were outside the scope of their authority, but were conducted under color of authority.
"He was not required to give an accounting of his business to the police officer"
That is correct. He did not give an accounting...and he was on his way.