As a matter of fact, the notion that God permitted it one time means we are then free to do it anytime reminds me of when my son was young. I had taken him out and taught him to fire a rifle at some targets. He was under direct supervision when we did it. This did not mean, which he discovered later, that it meant to could get the rifle anytime he wanted, without my supervision, and go out shooting.
Thinking that just because something was allowed one time under special circumstances means you thine have carte-blanc to do it anytime is a childish notion.
I think you're on to something here.
It appears that on several occasions in the OT, God permitted, even commanded people to break his commandments. For example, God said, "Thou shalt not kill." (Exodus 20:13). Yet, He commanded King Saul through the prophet Samuel to kill every man, woman, and child among the Amalekites. (See Obedience is better than Sacrifice, from TorahBytes.org.)
Why would God ever allow "exceptions" to the rules, even for one individual or group of people? It doesn't seem logical to me that God would contradict himself.
Religious leaders of all stripes have used the idea of "exceptions to the rule" to justify myriad practices, which "normal" people find abhorrent.
Those leaders will be held accountable before God, and He will deal with them in his own way.
What baffles me is how religions manage to survive despite the sins committed by their leaders and practitioners from the past (and present). History is rife with examples of sexual exploitation of innocents, power struggles, deceipt, and other atrocities among the religious elite of all denominations, yet Catholicism, Protestantism, and Mormonism have all managed to survive.
One question might be, how do we reconcile the "good" done in the name of relgion, while acknowledging the "evil" that also occurs?
Another relevant question might be, should modern day adherents of a particular religion be held responsible for the past practices of their founders? To me, the answer is Yes, if they continue in those practices today. We have an example of that with the current situation in Texas. The FLDS continue to live in their past, while the mainstream faith has moved far beyond it.
Has God "changed his mind" about certain practices? Perhaps. Only He can answer that. Have religious leaders "changed with the times?" Absolutely. One only has to glimpse into many of today's liberal "Christian" churches, which preach environmentalism, feminism, almost anything but the fundamentals of Christ's gospel to see that. Not a single denomination exists today in the same form as it did in the 19th Century (well, maybe the Amish).
Are modern-day Jews to be vilified because some of their revered prophets from the past condoned mass murder? Probably not. Do some Jews believe they should take the same course of action today (with regards to their "enemies" in Gaza for example)? Perhaps. Does that mean God will support them if they annihilate their neighbors as their ancestors once did? I seriously doubt it.
So where does that leave us? Mormons need to deal with their past. Not an easy thing to do given the numerous problems in their history. Others need to deal with the Mormon's "present." I think that is just as hard to do.