That sounds like a comment from a reporter who knows absolutely nothing about the park service, and nothing about NPS law enforcement in particular.
We used to joke that the NPS motto was, "Make no waves," and there is a lot of truth to that. There is probably no way quicker to get negative attention from management than for an LE ranger to take some enforcement action that draws public attention. "Warn politely and dismiss" is a way of life in the NPS.
A more likely explanation -- if reports of Springsteen doing shots with visitors right in plain view of the LE rangers are true -- is that they felt they had no choice but to take action. In other words, he was daring them to do something.
I found one article from Patch where they got the reports and verified who arrested him. I think it really depends. LE rangers wear the same uniform and are still considered the friendly face of NPS and not enforcers. I've heard that they have more resource protection duties, even in places where their jurisdiction overlaps with US Park Police. I found it interesting that there was confusion over who actually arrested him. The USPP union wanted to make it clear that they weren't the ones responsible. My west coast experience is that they weren't necessarily jerks about anything. There was this big event at the Presidio of San Francisco, and unless we were actually blocking traffic or a fire hydrant, USPP didn't even seem to care that we were parked at red curbs.
Sources told The New York Post that the incident happened when Springsteen made contact with fans. "Bruce stopped, took the pictures, then a fan offered him a shot of liquor, which he took while sitting on his bike, which was stationary .... park police saw what happened and they immediately pulled Springsteen over as he drove away."
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In a statement forwarded to Patch, Chairman of the United States Park Police Fraternal Order of Police Kenneth Spencer clarified that the arrest was made by National Park Service park rangers. "The United States Park Police was not involved in any way," Spencer said.
I've talked to police about the spirit of the law. We had block parties during National Night Out and our Chief of Police came to our neighborhood. Our tables were set up on the street and there was clearly beer and wine being consumed. At another even I specifically asked his successor what the deal was with alcohol and block parties. He flat out said that it's illegal. But they looked at the spirit of the law and wouldn't normally enforce unless there was a problem such as rowdy behavior.
I'm not sure what was the situation with Sandy Hook that they decided to ban alcohol altogether. Around here it's well known that NPS areas are where one goes to consume alcohol when it's not allowed at a city park. I was walking around with a Corona in front of the Walt Disney Family Museum and nobody cared. However, various superintendents have banned alcohol at places where there's been a lot of trouble such as Ocean Beach or other areas.