Those rules are for extremists, cultists who recide in big, multi-religious cities, where they are usually banned.

Or at least thats my headcanon, how else would Shar have as many followers as she does? Nobody would throw away a priest who drew followers to their cause just because the priest didnt believe in the sacrificing part. Of course we as players know the meta that Shar is evil and will corrupt most who follows her to eventually do bad stuff, but people in game doesnt care about religious politics and every religion calls out the other religion for walking the wrong path. Townspeople might form a small cult of Shar in a small town just because they were in grief for something and she’s the one who promises to remove that grief. Over time that becomes a bigger church. And an easy recruitment center for extremist Shar cultists.

This makes for better stories than “All priest of Shar has to kill a Selunite”, imho.