Is it legal ?
Yes why wouldn't it be ?
Passengers enter into a contract with the carrier. The contract is either the ticket and the terms contained therein or in the case of electronic bookings the terms you placed an applicable tick against, to show your acceptance. Most contracts are written by, and to protect the vendor, and hence it is very unlikely to be even handed or provide for reciprocity. Lack of balance, reciprocity or perceived fairness does not make a contract "unfair" in a legal sense. Contracts must be governed by the overriding law in the applicable jurisdiction or they may not protect the client (in this case the airline).
In this case the vendor is a point to point airline whose contract of carriage (read section 9 of their conditions of carriage ) makes it clear what their contractual position is in the event of a diversion. For the avoidance of ambiguity, read carefully the sentence in that section that starts with the word "unless".
I do not work for this particular company, but that would appear to be the part of the contract applicable to this question.
I cannot quite see the fuss about this issue. If an airline diverts and then without deplaning the passengers intends to carry them on to their contractual destination what right would the passenger have to vary that contract without possibly being liable for the repercussions ? The contract makes clear the airlines responsibilities but it does not allow for an individual to disembark en-route at that individuals whim. It does however allow a Captain to disembark an individual or to make a variance for the purpose of safety, flight regularity or the compliance with any applicable law or regulation.
The diversion may in reality be a more convenient destination for a passenger, but that is irrelevant. The contract (in this case) does not necessarily allow for it. The airline might as a result incur delays and additional charges for such a variance (damages), and the applicable route licence may not permit such action outside of an overriding "emergency" such as passenger illness or disruptive behaviour etc.
To the question, "What right does a Captain or the state have to prevent a passenger leaving an aircraft which is on the ramp?? " I would think the Captain and the State have many rights in this regard. Those that are applicable would presumably depend upon the situation and the jurisdiction. A couple that sping to mind are the UK air navigation order , The aviation and maritime security act (UK).
The statement, "I assume that if for some reason FR are unable / unwilling to take passengers to their designated destination, that the company shouldtransport them, regardless.....an unfair contrract is not enforcable under english law, but then Irish is a different ball game....it takes two to tango.
I would call it a hijack, what would you call it....????"
is from a common sense viewpoint, a little tortured, but again read the contract on their website. It is clearly not a "Hijack" as the captain is de facto "in charge" and has not seized control nor taken the passengers as hostages. He is complying with applicable regulations and the terms of carriage enshrined in the contract between the company and the passenger. I am not sure why this contract is unenforcable or unfair. Has it been judged as such ? Once again reciprocity is not a legal requirement unless it is a requirement of an applicable statute.