I am not sure I would llike to be "protected" against myself because of my bad temper and unability to mange staff. I would consider that a rather harsh rebuff.
You can also interpret it as a way to protect her from further comments. MT is a passionate woman who can achieve great things when she wants to, unfortunately it seems that at time that same passionate nature has a less pleasant outcome. In future she will not be able to make such calls and is by that protected from the flaws in her character.
By this means that the Grand Duchess has no manegerial function or any rights about any staff. Her function is purely representative, which must be seen as a rather clear (and deserved, it seems) rebuff of Maria Teresa.