I would suggest that there has to be some recognition of the relative scope and maturity of a country's economy and infrastructure when setting carbon emissions reduction targets.  It isn't necessary for all emitters to be on the exact same schedule to mitigate climate change as CO2 stays in the atmosphere for decades/centuries.  Given our advanced technology and the fact that that the US is responsible for nearly 30% of historical human caused greenhouse gasses it's not unreasonable to expect us to be able to reduce emissions more quickly.