I agree, most Mormons do not understand or know the weird stuff that is at the heart of their religious beliefs.
On the outside, their religion can seem so fluffy and can leave out so much of the Joseph Smith (and the rest) stuff, that it can look and be just like a soft, watered down form of Christianity. I know what I am talking about, I live right in the middle of them (well, sort of, I am next to one of their world hqs and surrounded by RLDS).
Christians should note that, even though Mormonism is not Christianity, that many people who call themselves mormons are fooled into believing the truth. The RLDS pastors around here have gone so far out of their way to look like Christianity that I believe it backfired on them. They are now getting people saved! Some of them are in turn becoming pastors. Keep in mind that this is RLDS, not Mormons exactly, but I think it is seeping into the LDS too.
This might be a good thing, if it keeps going that direction, but I suspect that it might lead to confusion. But when I look around here, at the RLDS church members, I think that they are people who are just trying to do the right thing. That does not make them good necessarily, nor Christian, but it helps.
I am not sure how much of this is getting into the LDS.
To sum it up, in my opinion: Because they just want to be considered "good" people.
In response to the additional details:
Really? They sure do a great job of appearing not to know the Bible, or even their book of Mormon very well. Is this a trick of some kind and they really know their stuff? Or do they still have a long way to go after they were a missionary or preacher?