No one has suggested that. You, however, have suggested that there is no point in the US lowering its emissions because China emits more total than does the US.
The atmosphere does not care about arbitrary political boundaries. There's a total amount of greenhouse gases we can put in it consistent with keeping global warming under any given target. A ton of CO2 from a person in China is the same as a ton of CO2 from a person in the US as far as the atmosphere is concerned.
So if a random person in China being responsible for putting about 8 tons in per year is unacceptable, then by what logic is a random person in the US putting in over 16 tons a year fine? By every physical measure, that US person is doing at least twice the harm to the atmosphere.
The US needs to be the leader on this, not making excuses not to reduce its own emissions.
The reason that per country emissions are important is that countries are convenient political and demographic units.
Of course, the West is already on that downward trend and China is soaring upwards with no signs of even plateauing. Within 10 years China will be technologically independent of the West and the West will have even less influence over its environmental policy than we do now.