I can just imagine, on some dark & stormy night when the power has failed at around 2am, an unknowing person gets out of bed to discreetly use the toilet without bothering to turn the lights on; flushes the toilet and .... SUCK SSSHHHHhhhh ...... not for the faint-of-heart! lol!
Hopefully, the system has a functioning check valve; maybe two; to prevent such an occurence!