It would of course be ideal if U.S. supported it as well. No doubt about that, however rather than being part of it and arguing about everything it is better if it is not part of it at all.
Over 95% of world's countries signed the Paris treaty. One thing to keep in mind is that not a single country in the world is even close to a situation where it could afford to go against the rest of the world in the long term in terms of its economy, population, land area etc. Both U.S. and Europe are getting less and less significant, no matter whether we like it or not. India's population increases by 16 million every year, in some countries every woman has on average 7 kids, so the population distribution is changing rapidly and these countries are no longer so under-developed, in fact quite the opposite in some cases. As I mentioned above, this is a problem of Europe as well, but it means we should even more try to co-operate with each other.
Back to my original point, I am not contradicting myself. I still say it would be great if the U.S. was in, but if this would mean only problems then better if it leaves.