My take? This is really a pretty simple equation for FB. Politcal ads = good business opportunity ....perhaps essential.

Since FB tends to be a self-reinforcing echo chamber on both ends of the political spectrum, political ads will keep users engaged with the platform. For a variety of reasons interest in the 2020 campaign is likely to be sky high so allowing the ads should keeps FB's DAU's & MAU's in the preferred direction. Not allowing the ads could in-turn adversely affect those metrics and subsequently pull down ALL of their advertising rates. Freedom of Speech rational of allowing adds? (scoff, scoff, snort, outburst of laughter.....)