Comment 7 for bug 901498

set_up_oops_reporting does this:
oops_observer = OOPSObserver(errorlog.globalErrorUtility._oops_config,
    log.PythonLoggingObserver(loggerName=name).emit)

to deliberately pass non-error messages onto the regular LP framework logging setup.

So changing the logging setup for twistd apps to never ever use stdout/stderr will fix this.