I think the reason is because it is a vertex group essentially. So it is consistent with vertex groups, shape keys, UV maps and anything that is assignable to a mesh.
These are not objects that are freely movable in the scene. They are stuck to the mesh.
So think of all data as having a level. And there are two main levels.
Vertex Groups, UV Maps, Shape Keys, Materials and so on are all inextricably linked to the data mesh.
Materials, like Surfaces in LightWave have the additional unique property that they are also a data block that is portable. Meaning that it exists on its own. So it is a sort of bridge across objects. But it is the only Mesh level data block that does this. The renaming of Surfaces, as far as I remember it, works basically the same as LW.
Texture slots actually work in the same way as Materials with the added feature of being able to stack textures and this is like the layers system in LightWave.
But if you are going to layer textures it is probably more advantageous to use Nodes.
The image management method and interface I feel really could use a lot of help. And I agree with you there. In fact I think a lot of things need improvement interface wise.
My focus becomes simply. Illogical or strange as it might be - just figure it out and move on… lol
So while I feel your pain, all I can do is point you in the direction of what is, and try as best as I can to also impart what logic exists behind it from my experience. And also note this is not just me thinking about it. But by experience, I mean that literally using it for a number of years and seeing from a practical level how it works and why.
This is in no way an assertion I think all of it is fully baked yet.