I don't know what you mean by "chosen/assigned". As far as I know, there has been no official declaration stating that we are to keep the genealogical record for the US. However, if such a thing had been done, I would say that it was done as a convenience. Because of our belief in the importance of trying and sealing family lines together as far back as we can, we maintain the worlds largest data base of genealogical records. It's not just limited to the US. We are gathering all the records we can, digitizing them, and then making them available to anyone who desires to trace their ancestry. (Not just Latter-day Saints.) Since we are going to gather this information any way, it might make sense to let us and just use our central database rather than try to maintain another one. However, I don't really think that this is what is going on. The US Census Bureau is charged with maintain most of the statistical records regarding the population of the US. Personally, I doubt that the bureaucrats there would want to give up their jobs by turning that responsibility over to a private organization. Then of course there are the various state and local organizations that also maintain such records. Personally, I am more inclined to believe that this is another one of those rumors that is started by our enemies. It is a common theme among some of the more rabid of these people to take little things, blow them all out of proportion and then say that it is evidence that we are trying to take over the world and force our beliefs on everybody else. Many may be thinking I am paranoid here. However, I can assure you that I have read quite a bit of the anti-Mormon literature and in many cases, this is a very clear tactic of theirs.