Hmm, I don't think I can agree with that statement either, sorry. They did not have sufficient cause for deliberate detention because no threat was communicated. I believe if the opinionated traveller pursues the issue properly, there will be a likely discplinary act promulgated on the supervisor. I've seen my share of supervisors get sacked too; they are also easily replaceable.
Unless you are judge sitting in a courtroom, I don't believe anyone gets to detain someone just because they don't share the same opinion. In a courtroom, that would be contempt, but that is the exception to the rule. (Well, except for when you mouth off to the DI that is, then you get detained in the forward leaning rest position for a while).
_________________________
The ultimate result of shielding men from the effects of folly is to fill the world with fools.
-- Herbert Spencer, English Philosopher (1820-1903)