I suspect this one will have to go to court and probably to appeal to get a definitive answer. A civil contract is not valid nor enforceable if it involves a criminal act. If a dying man enter into a contract for someone to help him to die then an offence would still take place and the contract would be held invalid.
If a person contracts themselves or others into what is effectively slavery, then that would also be held invalid.
Just because you have a contract term which says that you can be detained does not make it right to unlawfully detain someone, and therein lies the problem.
Are they acting reasonably under the circumstances and within the law, or are you being illegally held against your will without due cause. The court's interpetation of what is reasonable or not is open to question, I suspect they might accept the security aspects of the situation or immigration's refusal to permit entry but saving on baggage handling cost is another matter.