This is indeed a feature of our control suite. It is most often used for buttons, which, when dragged into a pannelbar (and thus becomng its children) have to change their back color to the back color of their parent. In the case of the textbox, this feature is not as useful, and might indeed be removed in the future (for the case of the textbox).
If you'd like to share more details on the scenario you're working on, and why you need to place a textbox within a label, we could try and find a workaround to get the behavior you're aiming to implement.
the Telerik team