When the RadNumericBox has focus it displays a clear button on the right, the "X".  In a high data density screen or grid, space can be at a premium and this button takes space, and when the user has a keyboard is not necessary (they can use the delete key) It would be space saving if this button was optional (as per the WPF version).

When you are in tablet mode and try to type 6 from the on-screen keyboard - the "-" sign is applied.