Rio Declaration contains references to common but differentiated principles. Then, what is the concept of this principle? How should we understand it? There is another instrument called "agenda 21", which is made in the U.N. conferences on economic and environment on 1992 Rio conferences. These kinds of declarations are soft law, without any binding force. But states would like to regared them as a direction for their behaviors. 

Question:

Why do states in international community obey the principles without bingding force in international environment law?