Calling Method "Abort" on PWO over REST

I am developing a business process interface based on the REST interface that was introduced in Q1IM 7. The version we're using is Q1IM 7.1.

I can call methods in genral just fine, but the method "Abort" on a PersonWantsOrg object always fails with the all to familiar exception:

[2133323] This method can only be called internally.
 System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. ---> VI.Base.ViException: This method can only be called internally.
   at QER.Customizer.PersonWantsOrg.Abort(ISession session, IEntity entity, String strMessage, String identPwOState, String uidJustification, CancellationToken ct)

Well, how is one supposed to cancel an active order otherwise? I am aware that I could write a script that does just that and call it over the REST interface. But that's hardly practical.

I wonder why calls over the REST interface are not supposed to be "internal"?