I still think of some of you are missing the point.
Nobody is "forcing" passengers to be detained against their will. This is a case of contracts, efficiency and regularity of air navigation. Flying is not usually analogous to jumping off a number 7 bus. That is you can disembark at some point other than your original destination because it suits you to do so. If you are contracted to fly from A to B and the aircraft has to make a stopover in point C, you cannot elect to disembark at that point simply because it suits you to do so. There are many examples of routes particularly in the United States, but in other countries as well where a scheduled stopover is made, however passengers are paying a cheaper fare to fly beyond that stopover than they would if they had contracted to it in the first place. In such circumstances anyone disembarking to avoid the higher fare would be in breach of their contract and likely charged accordingly.
In this example the airline has carriage rights between the origin and destination and has diverted for extraordinary considerations. To allow a passenger to disembark for anything other than accepted "emergency" considerations might place the carrier in violation of its traffic rights. It would almost certainly involve the carrier in additional delays and extra costs. The passenger has no right to place the carrier in such a position and failing any extraneous reasons, would be in breach of contract and possibly be liable for any damages that may result.
This might not be an obvious consideration to some people and the Captain has a duty to point this out as he sees fit or in this case is advised. That is not an illegal detention.
A.viator I think you glossed over this sentence .
unless the aircraft continues to the original destination. You might also care to have a read through
this as it provides in conjunction with the Tokyo convention on these matters even more "food for thought".