It is unnatural since under normal circumstances the female dog has a muscle in the vagina that holds the knot. Since a woman is not equipped with this tool, they can not hold the knot in. You should not be holding the legs, you should hold the knot, but most people do not have any idea how canine anatomy works in this regard, so they hold the legs or the tail.
I usually leave such videos alone as long as the dog look ok, since if we were to remove that, the porn section would be mostly empty.
But I do remove/report those where the dog appears to want to leave and is held in place despite that.