It does here.  Are you sure it acts differently from C-M-\?

(While trying out, I found that `indent-region' doesn't always indent,
but its docs (C-h k ESC C-\) tell you how it determines if and to where
it indents.)

Actually, you can force it to indent using a prefix arg:

 C-u 4 C-M-\      ;; indent to 4th column
 C-u 4 C-[ C-\    ;; ditto
 C-u 4 ESC C-\    ;; ditto

Bye,
Tassilo