For some reason, if I use a .tga with alpha channel (that has Black and white spots), that alpha channel affects the Solid viewport (Viewport shading > Color > "Texture"), even though it's not even being used in any nodes. See no way to turn this behavior off. Material is set to Opaque. I would like some kind of answer, even a "that's just the way it is" coming from a knowledgeable source (i.e. the devs just coded it this way, without any way to disable the alpha affecting the Solid viewport). .blend file here
The Texture option in the Solid viewport shading is showing the texture - not the material. If you are using the alpha channel or not is a matter of how you setup the material. But the texture itself is a PNG which has transparency, and so it is shown with transparency if you set the option to Texture.
And since it does not show the Material Output, it also does not show for example a combination of different textures if you mix them in your material - it shows the image of the Image Texture node which is currently selected in the Shader Editor. If a non-texture node is selected, it shows the image texture that was last selected. If a non-image texture node is selected (like Noise, Voronoi etc.) it just shows the color set under Material Properties > Viewport Display.
By the way, to show the Material Output you have to use the Material Preview or Rendered shading mode. The Material option in Solid shading does not show it, it shows the color which is set under Material Properties > Viewport Display like described above when you have a non-image texture selected. The Object option shows the color which is set under Object Properties > Viewport Display.