It is incorrect to assume that a sub would not allow herself to be subjected to such nonsense. Someone who is new, has little to no experience, and has been told to "submit" in any form, who hasn't had time to grow into her submission, who has read assorted fantasies and doesn't know the realities, etc. or someone with low self esteem would/could easily fall into this trap. At that point they could be turned off entirely by the entire concept of submission due to the confusion and mistrust created by such a scenario.
As stated before there is a different between being disciplined for a wrong and punished in play. And as stated before that is a fine line that has to be made very clear. That seems to be part of the equation that is being missed. None of this is a pretense for some. If that works for others that is fine, as long as BOTH parties are aware of the rules and how the "game" or roleplay at that point is to be played out. Otherwise, actions such as this can and does cause harm to many a good sub.For the same reason that a master might punish a slave for sluttiness even if he was the one who told her to dress that way. Because it's part of an elaborate pretense, the goal of which is to set the stage for discipline as pleasure.