You need to be authorised to run ads about politics, so this is simple way for them to know if it's a political ad or not. The challenge is if advertisers miss classify what the ad it about, which then is subjective to Facebook as you say.
But all facebook cares about, is not to be blamed again for fake news on their network are responsible again for a not desired outcome of the election. So this is just one step towards that image, nothing more.
