What does ""the right of every man are diminished when the rights of one man are threatened"" mean

Excerpt from momotaro, or "little peachling" japanese folktale momotaro finding that he excelled every body in strength determined to cross over to the island of the devils, take their riches, and come back. he at once consulted with the old man and the old woman about the matter, and got them to make him some dumplings. these he put in his pouch. besides this he made every kind of preparation for his journey to the island of the devils and set out. then first a dog came to the side of the way and said; "momotaro! what have you there hanging at your belt? " he replied: "i have some of the very best japanese millet dumplings." "give me one and i will go with you," said the dog. so momotaro took a dumpling out of his pouch and gave it to the dog. then a monkey came and got one the same way. a pheasant also came flying and said: "give me a dumpling too, and i will go i along with you." so all three went along with him. in no time they arrived at the island of the devils, and at once broke through the front gate; momotaro first; then his three followers. here they met a great multitude of the devil's retainers who showed fight, but they pressed still inwards, and at last encountered the chief of the devils, called akandoji. then came the tug of war. akandoji made at momotaro with an iron club, but momotaro was ready for him, and dodged him adroitly. at last they grappled each other, and without difficulty momotaro just crushed down akandoji and tied him with a rope so tight that he could not even move. all this was done in a fair fight. what might reasonably be assumed about traditional japanese culture from this portion of the passage?