Roosevelt did not allow Japan to continue the genocide. That is the point. Stopping the war in China was not really an option for Japan. Technically, they could have, but everyone knew they would not. The actions of Roosevelt resulted in the Pearl harbor attack. That is exactly why it was done, and that is a good thing. Roosevelt wanted in to the war, but the US public was not interested. By forcing Japan to attack Roosevelt could make the US population interested.

The only way to get the US into the war with a motivated population was to make someone punch the US. The only one that would and could hit the US was Japan and their hand could be forced.