Offload is used in the cases you mention to distinguish the action taken from the more commonly used disembark.
Disembark is the normal word used for people getting off a conveyance (originally ships, but extended to airplanes, trains, buses etc.) but using disembark is a little bit ambiguous.
Jane Doe was disembarked from the 9am flight this morning
Jane Doe was offloaded from the 9am flight this morning
They both mean the same thing, but as it's normal for people to be disembarked in a non-forceful way, using offloaded for a passenger makes it clearer that the disembarkation was against her will rather than the normal procedure.
OED does mention that off-load has been used for people.
off-load v. orig. S. Africa (after Du[tch] afladen)
trans. To unload. Also transf. and fig., to discard, get rid of, relieve oneself of (a person or thing). Hence off-ˈloading vbl. n. and ppl. a.
1952 C. Day Lewis tr. Virgil's Aeneid v. 113 They enrolled the women for the colony, off-loaded the men who wanted To stay there.
Also compare the definitions from The Free Dictionary and note defn.3 for offload.
Disembark
v. dis·em·barked, dis·em·bark·ing, dis·em·barks
v.intr.
1. To go ashore from a ship.
2. To leave a vehicle or aircraft.
v.tr.
To take ashore from a ship.
http://www.thefreedictionary.com/disembark
Offload
v. off·load·ed, off·load·ing, off·loads
v.tr.
1. To unload (a vehicle or container).
2. Computers To transfer (data) to a peripheral device.
3. Informal To get rid of and pass on to another
v.intr.
To unload a vehicle or container.
http://www.thefreedictionary.com/offload