The NPS has the same rules for the National Mall where I've filmed several times, and I've never need a permit. And the National Mall has far more police presence; so, you can feel pretty assured that they aren't arresting or suing everyone who sells a picture of the Lincoln Memorial. The rules actually state:

"Filming/Photography Permits are issued for photography, filming, and associated sound recording to ensure protection of resources, to prevent significant disruption of normal visitor uses, or when they involve props, models, professional crews and casts or set dressings. Permits are required for access to areas normally closed to the visiting public."

The artist in question doesn't violate any of those elements. If you look deeper into the rules for a specific park, this is what you'll fine. You need a permit if you are violating the spirit of the park, or if you would create a significant distraction. Filming a chase scene down the Canyon, you'd need a permit...taking some pictures, you're fine. Those rules are more designed for a crew like National Geographic would might be bringing in 100's of lbs of gear to shoot, or who might be trying to access restricted areas. Or to film/documentary crews who want to use the area, but will similarly be bring in crews that would have an impact beyond normal visitation.

Since I haven't read anywhere that they are sending cease and desist letters to the thousands of photographers selling Grand Canyon photos, I'd say he's pretty safe.