Cant help but think someone on their cult simply diverted their religion to that point(i mean,one of the things Quetesh represents is life and fertility),and when Ziva was about to punish MC for making them break their rule,she got herself reprimanded in turn,which makes me think that one of the 2 things created that rule:
1)Protection:to avoid the cult to bring in people infiltrated for their rank,and to avoid their ranks to grow to big to be controled properly
2)Sabotage:to make their numbers too few for them to outfight possible coups or attempts to take over their cult
Considering how easily their cult was wiped out with Nergal curse i supose that it was the second one(or maybe Quetesh is far to be brightest tool in the shed,and the butthurt Nergal used her own rules to give her the ultimate Middle Finger)