First, it works exactly like the WinForms ComboBox control with default settings. If you drop it onto a form, its DropDownStyle is set to 'DropDown' allowing you to enter arbitrary text or select it from the list. If you want only to select from a list, you must change DropDownStyle to 'DropDownList'.
Second, look at this from the point of view of 'adding features to existing ones'. If you have a text cell and adds a drop-down list, you ADD something to the existing functionality and this should not disable anything you had earlier. In other words, what you already have - including text editing - must work after adding a drop-down list until you disable this explicitly.