No response I give will justify this policy, and trust me when I say this is a hard topic for me personally, both to understand, and harder still to explain. The answer I provided is only what I found appealing, and it was from a black member. Yes, it refers specifically to the black population, and yes, I feel they'd already been through enough. However, someone else felt they perhaps needed additional strengthening and the thought appealed to me. As far as societal pressures and prejudices affecting even prophets, if you are religious at all (you in general, as I know you specifically are not), you recognize that there are scriptural practices and prophetic direction that are completely unacceptable by today's standards. I don't know why, but perhaps God allows people to be faulty and unjust, gently guiding them along their way until they come closer to perfection as a people. Perhaps such is the case with this church.
I encourage you to check out those links, particularly the Genesis Group and blacklds.org, as both would give you a better perspective from those far more directly affected by the policy (I wasn't even born when it ended).
As for polygamy, as I've stated, it has bee more of an exception than a rule. The Book of Mormon, published prior to the practice's introduction, condemns polygamy with force, and then leaves a tiny little caveat. God's laws are not that polygamy should always be in place except when illegal. God's law is that polygamy should only be in place when prudent. Thus, its removal indeed complied with federal law and prevented serious punishments, but it simultaneously was a return to standard practice. And bear in mind that polygamy is not illegal in many countries such as Mexico. However, if a Mexican Mormon is found advocating or engaging in the practice, he/she will be excommunicated.