They are confused! They already have trouble conceiving of a religion that is non-theistic, so when they encounter one that is optionally theistic it gives the Mormon elders brain spasms.
My son and several members of his troop were raised in the Unitarian Universalist church so I got firsthand experience with this phenomena ;).
Incidentally, UUs generally have more problem with the BSA than vice-versa, because of the aforementioned cluelessness, and UU congregations sometimes encourage local dissent inside the BSA, or sponsor alternatives like the Spiral Scouts.
