I doubt this activist was removed because of his race, colour, religion or national origin either. He didn't mention any of these reasons in his blog posts.
I was merely refuting the statement that any private property owner can have anyone removed without cause. They cannot. There are limits. The Civil Rights Act is just one example of such limitations. It was not meant to be an exhaustive listing of all the restrictions on the authority of a private property owner's authority to have people removed.
[ETA: In particular, you asserted that something about your ticket agreement gives the police the authority to remove you from a sporting event for no reason whatsoever. That's not true. If you're doing something wrong--creating a disturbance, throwing bottles, otherwise breaking the rules--that's a situation easily and starkly distinguished from the present situation.]
Uniformed police officers are acting in their official capacity even when their time is paid for by an event organiser.
Yes, but the officer fully admits he was acting at the request of his client (the Romney campaign) in asking Bieber to leave. As I've said, arguing that the arrest was anything but an extension of this action (in the absence, for example, of any disruptive behavior on Bieber's part), doesn't hold much water.
Again, I think the arrest was at least problematic.
I am still having trouble which aspect of their authority you're questioning. Is it that they had no authority to remove him from the premises? Or is it that they had no authority to arrest him after he left the building?
They had the authority to arrest people, but no cause to invoke that authority wrt Bieber. They have the authority to ask him to leave at the request of the client. However, they do NOT have the authority to arrest someone at the request of the client.
I've made this clear. I'm not sure why you don't understand the issue.
That means any action they took would be under the authority of the police department and not the Romney staff.
It means no such thing. No one has asserted that the police took it on themselves to ask Bieber to leave. They were instructed to do so by the campaign staff.
The reason this story isn't growing legs is because the officers acted under their own department's authority.
That is false. They acted on instructions from the campaign staff in asking Bieber to leave. The officer admits that he saw Bieber doing nothing that warranted removal.
It's noteworthy that he hasn't even posted a copy of his arrest report.
You're willing to make something of the absence of an arrest report, but don't find the campaign's refusal to comment at all telling? First, there may have been no arrest report. Second, Bieber might not have it, or it might not yet be available.
ETA: And Bieber's blog already indicates what his first step was. He wrote a letter to the Romney campaign and said he will report any response he gets. The Nashua Telegraph got a refusal to comment when they tried to get the Romney campaign's side of the story.