Diversion
A LH flight from Frankfurt to San Fransisco diverts to Oakland 12 miles away on the other side of the bay. PAX kept onboard and not allowed to disembark. Why? LH has no traffic rights to OAK.
Just a week or so ago most of Persian/Arabian Gulf airports where closed for hours due to fog aircraft divert all over. Singapore Airlines SIN-DXB flight diverted to Sharjah less than 12nm away from DXB. Again PAX where kept onboard for the reasons allready mentioned such as lack of groundhandling equipment, facilities and most important: Traffic rights to SHJ.
There is a big difference between words "Keep" and "Detain". While diverting you are keeping PAX onboard, certainly not illegal, and has nothing to do with a detention as described in various laws around the world.
Furthermore captain/commander does have the right to "Detain and hold" a PAX suspected (Probable cause) or caught in criminal activity (Unruly, violent etc) onboard until such time the PAX can be handed over to the authorities.