UA, in theory, blocks the adjoining seat for Premier-level members.

If you are not a Premier-level member, then those seats will not be available to you at time of booking.

Also in theory, I believe a Premier-level member is able to reserve any seat on the plane via the seatmap, but I too have noted that often an aisle/middle or window/middle seat will be blocked, so perhaps UA's computers do it for any status level.