You said: "This is untrue. There are facts of non-married individuals that need to be shared."
What is untrue? A "shared fact" can only be implemented between two people that form the parent portion of a "family unit". In general (traditionally) this means a husband and wife, but is also extended to a non-traditional couples, including unmarried but only when they are the parents of a family unit.
I realize that "facts" are shared among all kinds of people, but the database structure only supports the parents of a family unit, not facts between parent and child, grandparent and child, brother and sister, etc. Your example of "mother moving in with a daughter" is an individual fact that you want to "share" but in FTM this is not a "shared fact". It is two separate facts. As silverfox pointed out you can copy a fact from one individual to another.
FTM did a poor job of "titling" this concept. They have confused more people that you can count. Some programs (not FTM) do allow for facts to be shared when you enter them between any number of participants.
Please note: I am trying to be sensitive with a term that is very hard to explain. By "parents of a family unit" I am speaking to the database that is driving the design. In general the database was initially set up to only support married individuals one male one female, the concept was expanded to eventually include same sex partnerships, and non marriages. But the underlying structure is still a family unit, it is within this family unit that two people (only two) wife and husband that a fact can be shared.
No flames please I am explaining something that was not my design but was designed by LDS.