Because:

  1) The __debug__ flag is defined to be process-global.  If you test it in one module, your code should be able to assume that it has the same value in all other modules

  2) python-dev does not support running .pyo code without -O turned on.  In the future it might be the case that -O would actually change the behavior of the running python interpreter such that .pyo code would fail of -O was not on.

So, you can do the rename or importer trick if you want, and it will work right now as long as your program does not depend on __debug__ being globally consistent (which would be the case for almost all programs), but we do not guarantee it will continue to work in future versions of Python.