Materials with textures are given an opaque white diffuse colour to avoid any undesired shading effects arising from Blender's default off-white Base Color.
However this feature has never actually been implemented—the diffuse colour is set from the (hidden) Base Color and Alpha inputs when an Image Texture node is connected.
The README states that:
However this feature has never actually been implemented—the diffuse colour is set from the (hidden) Base Color and Alpha inputs when an Image Texture node is connected.