Because its optional – on the Paris Climate Change Treaty Politico
Obama administration lawyers discovered early in the day that the latest draft text had a potentially deal-killing tweak: Deep into the document, in Article 4, was a line declaring that wealthier countries “shall” set economy-wide targets for cutting their greenhouse gas pollution.
That may not sound like such a headache-inducing roadblock, but in the world of international climate negotiations, every word counts. In previous drafts, the word “shall” had been “should” — and …”shall” implies legal obligation and “should” does not.
Notably Arabic and French dont’ have the same distinctions which apparently added half a day to negotiations.
For the same reason never use ‘will implies some point in the future not at the moment of decision.