Of course, you are right. You can't force anyone to do anything.
And yet, remarkably, international cooperation on many issues still exists. Practicality come down to being diplomatic and being compromising.
You are right - it is difficult many groups of people (e.g. Dominionist Christians that think the rapture will happen in their lifetime) to even understand the need for policies of long-term sustainability.
How would you force China, India and Russia and other pseudo capitalist countries to pay, let alone the United States, Europe etc?
If the UN enforced a fee of some sort, why wouldn't we just offshore space launches to a country that ignores the UN.