It's a valid question and indeed one that many environmentalists seem to ignore. I think the answer is basically what Roland said in his long-winded way - it's not so much about a better or worse planetary temperature, as it is about the speed of the change and the immense costs of adjusting to those changes.

To put things in very simplistic terms just for the sake of argument: suppose that the world's total amount of habitable land and total agricultural production capacity remains the same, because while some arid places become complete deserts and parts of coastal areas become permanently flooded, other previously inhospitable places in, say, Canada and Alaska and Siberia, become much more attractive. So then we haven't really lost out on the whole, but the costs of relocating tens and hundreds of millions of people to different regions or in most cases entirely different countries, are still astronomical, not to mention the political problems linked to that, or linked to more small-scale problems like rivers drying up or diverting their course. If many people think that already the US and/or Europe are taking in too many refugees, just imagine what things would look like when there are tens or hundreds of times that number of refugees.

Even if you indeed assume that in the US the target would've been legally enforced while everybody else got away with reaching only, let's say, three quarters of their respective goals, that still doesn't say much if the targets of the US were more modest to begin with. It certainly matters what the numbers were.