The options for editing attributes in the attribute window change depending on the layer's symbology. Single Symbol uses a pull-down for domain values while unique symbol uses a button (ellipsis) that brings up a symbol selector.
The symbol selector shows what symbology is used for each value so that the editor makes sure they are picking the correct value. The drop down can still be accessed when editing directly in the attribute table.
Another workaround is to have two layers in the MXD pointing to the same feature class. One will be used for display purposes with unique symbology based on the field with the domain. Make sure this layer is not selectable to avoid accidental editing. The second layer will be for editing with single symbology to be able to access the domain drop down.