So Wardens and CT's are allowed to tell T's to drop their contraband however you have to call them specifically by name if there are many T's you should single them out by name. You can't force order "All T's drop your contraband" and kill T's who still has contraband. Reason behind this is because if a T drops their contraband on another T, the other T may not know he was dropped contraband and can get killed for unknowingly having it.
Generally speaking as a Warden you are not barred from saying "All T's drop your guns". Its a good way to scare them into dropping their shit however you should still individually call the T by their name to have them drop their contraband, if that makes sense.
EDIT: If a Warden pardons a rebeller he can also say along the lines of "If you want a pardon come back with no contraband" and generally speaking that has been allowed as well.
TL;DR: This rule has been kept vague because there can be many different type of scenarios and wording it specifically can cause issues. I would say its mainly admin discretion.