I would say that the responsibility to define a backend falls either on the Clojure interpreter or the user of the interpreter, then. Java is not typically run on an interpreter shell and the defaults that normally are sensible for Java probably aren't appropriate there.

It's a perfectly sensible default for a framework to warn you at runtime that it's not configured properly, and stderr is the appropriate channel for that to go out over. I would even go so far as to say that it should default to stderr output on warn/error messages unless explicitly muted, instead of merely switching to NOP output after the initial misconfiguration warning. An interpreter might prefer another default but I think that's probably the best default for most Java projects.

The way I see it here, Clojure built on top of a stack without understanding the caveats that go with the stack. It's simply a fact that Java logging is a mess, there are four major competing standards and you need to tell Java which one you prefer, because any project will eventually need to interact with all of them if it continues growing. It's a fact of building on that stack.