messages appear on stderr in batch mode

[Glenn Morris]

I never noticed before, but:

emacs -Q --batch --eval '(message "hello")'

prints "hello" on stderr, not stdout. This is a little annoying when
trying to write automated build scripts etc for Emacs, since stderr is
full of noise, and any real errors can be hard to spot.

I guess there is a reason for this, since it is in the manual (and the
man page, no less); but why?