United959 -
Perhaps the most straightforward way to explain the "joint" issue is by reference to community property states. In such states (e.g., California), divorce law generally treats the assets accumulated by spouses during the marriage as being owned jointly, regardless of which spouse has title. In such a situation, the judge might well rule that the miles must be split. Judges in other non-community-property states might rule the same way.
If UA says that UA's policies forbid such a split, the judge would presumably determine how much the spouse that keeps the miles must compensate the other spouse for the half of the miles that can't be transferred.
Thus, the question as to whether they can be transferred is valid and of some interest - perhaps to more of us than one might think!