[Silhouette]

Why is it implicit?

Because if we have module A depending on module B via some clearly defined interface, but in fact the behaviour of module B also depends on global module C that is set up elsewhere, then B's interface no longer fully describes what A can expect.

If C's scope were limited to what's happening within B anyway then this would just be an implementation detail. If C were given to B as some form of explicit dependency by A, then it would be a specified part of the interface. However, if C effectively has global scope by any mechanism then there is now an implicit interface to change B's behaviour that A doesn't know about. Developers reading or maintaining the code for A, C, and anywhere else that can affect C if it's mutable, then need to be aware of what each other are doing, and changes to any of these parts of the code potentially affect any of the others.

(I'm not going to address your other point, because I didn't say anything about service locators in the first place.)