The airline was right ...
I realise that this point of might not be totally popular among the punters, but this is a case where the airline had to step in to prevent worse.
You see, this particular passenger impeded the boarding process by importuning a flight attendant with a question. That alone is questionable behaviour because a smooth boarding process is essential to quick and safe aircraft turnaround.
But it gets worse. His alleged question was a clear attempt to take charge in a situation where, for security's sake, passengers must remain calm, passive, and compliant. It is in everyone's interest to reinforce this behavioural norm by removing troublemakers at the first sign of non-compliance.
And last but not least, we don't know the man's actual question, his body language, his tone of voice, or his facial expression at the time of the incident. How likely is it that this gentleman behaved innocuously and that the airline acted willfully in disciplining him? As the police officer who responded to the incident correctly noted, we only have his word for it that he wasn't asking if police were on board. We see plenty of individuals who display aggressive and abusive behaviour which they then hotly deny. Take the case of Naomi Campbell for example.
So, how likely is his story really?
Given the circumstances, all I can see is that this individual abused his freedom of speech to spread negative comments about the airline. Is there any reason he should not be sued for damages to the airline's brand name and reputation?