I think that it's basically a special case of how in those languages it's easy to assign a reference to things. Adding a list to itself is not really that different than assigning a reference to an object as a member of itself. For instance in python you can do:

class Node:
    __init__(self, value, next=None):
        self.value = value
        self.next = next

def main():
    head = Node("last")
    tail = head
    for i in range(10): head = Node(i, head)
    tail.next = head

And this creates a cyclic linked list. You could just as easily assign a = Node(9); a.next = a and have the graph have only one vertex. It's the same principle with adding a list to itself: you're not actually adding the list to itself; rather you're putting a reference to the container, inside the container. Like writing down the location of a bank on a slip of paper, then putting that slip of paper inside a safe deposit box stored inside the same bank.