The "legality" is possibly dubious. It is likely to be illegal to "hold" anybody on an aircraft, or anywhere against their will.
However, it is somewhat semantics. On a diversion to LTN (dest LHR) and Operator's plan was to take all to LHR ASAP i.e. not disembark anyone, no facilities were made available at LTN for passengers to disembark. One passenger tried the "imprisoned" trick on me, and I stated I was not holding him on the aircraft. The steps were there, and if he wished to walk down them, then fine. However, I would be calling the police to say he was wandering around the apron, sans ID etc., and he would no doubt get a night in the cells... and that continuing to LHR might be preferable. He chose to stay... And FR's security line probably corresponds to this.
On other diversions, it has been possible to disembark some passengers, especially if it can be established that they have no hold baggage. Bear in mind that disembarking the few can really scr*w up life for the rest in baggage, loadsheets etc.