Because we are trying to have a rational discussion. And it's possible that "Nations act in their self interest" is often wrong, or is not well defined.

For example, if a Guatemalan dictator commits genocide against its own people and is benevolent to the United Fruit Company ... is that "Nation acting in its self interest"? It's not a clear cut question. Equally it's not clear that the behavior of corporations, dictators or governments regarding copyright at different times in history is simple "self interest".

[0] https://en.wikipedia.org/wiki/1954_Guatemalan_coup_d'%C3%A9t...