There is a reason, and that is that it may break existing code in the field relying on the current behavior.  This is (unfortunately) true regardless of whether the function is public or private, though the fact that it is ostensibly private is likely to reduce the amount of breakage.  The fix needs to be evaluated to try to guess whether breakage is likely, and if it is, it would not be a candidate for backport to 2.6 or 3.1.