While it may be the individual that chooses to do the actions that they would be disfellowshiped for, it is the organization that carries out the punishment.
Otherwise there would be no judicial committees, no formal announcements, etc.
It is the punishment that is being debated, not the "root cause" of said punishment.
Ask the elder if the guy at the electric chair chose to be executed. Likely he would say that they did. In reality it is the judicial system that chose the appropriate punishment for the action. (this is not to debate right/wrong of the death penalty, it is simply an example).
For the witnesses it is simple black/white decision. They allow little grey area. They state:
"Whoever that is not with us, is against us", when in fact they should be stating " Whoever is not against us, is with us".
While it may seem to be the same statement, there is a subtle difference.