It's pretty easy (and a bit depressing) to see why: imagine seeing a headline that agrees with you politically. The candidate that you support did something good. Or the other side did something bad. It confirms, yet again, what you already know. But now, there is a little label that says that this may be fake. It's very easy to rationalize that the moderator is biased or doesn't know what they are talking or that this is simply what they want you to believe, isn't it? The bubble is real.
The status quo of Facebook is worse than them not touching any of the political content on their site.
> The second Facebook censors anything, they are immediately hit with a furor of complaint that, under those standards, they are obligated to censor some slightly less objectionable thing.
This is like trench warfare. Facebook never drew an arbitrary line: it just kept being pushed back by public and media pressure here and there, retreating in bits and pieces. Obviously if you just ignore that history, it looks like an arbitrary line now, but it was created by complaints almost identical to the ones you're making.
You are attributing very little agency to a company that makes its decisions unilaterally (sometimes even ignoring laws). This is a gross misrepresentation of FB's position. FB is not a victim here.