There is a recent PEP excluding IDLE from this policy, allowing it to get usability improvements on older pythons. One could think that enabling completions is a similar usability fix (though one change isn't worth the arguing to get exception) but in fact this change required a new var in `sys` and changed site.py, both unacceptable for a minor release.
That's even for backport to 3.3. As for 2.7, even when when allowed, most changes aren't backported to it simply because the code has diverged a lot and core devs are tired of implementing changes twice.
