@wrongwright, the button to the right on the snapping settings is ‘Align Rotation to Target’.
Turn that off and the line will not show.
Align Rotation to target can be used for snapping objects such as screws to align with the target objects face/edge/vertex. Instead of trying to manually rotate the the screw in to the correct position.
When snapping vertices there is not much point to it, you can turn it off or just ignore it.
