_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