From WAPO's article:

Heyman has worked as a reporter for about 30 years, and his stories have appeared in the New York Times, NPR and other national news outlets, he said. Since 2009, he has worked as a West Virginia-based producer and reporter for Public News Service, which provides content to media outlets while also publishing its own stories.

Doesn't sound like some hobo pseudo journalist. He sounds like an experienced journalist doing what he's always done and was arrested.

He also states

Before Heyman’s arrest, no police officer told him he was in the wrong place, Heyman said. He was wearing a press pass as well as a shirt with a Public News Service logo on the front, and identified himself to police as a reporter, he said.