Facebook is not Old Navy (one of thousands of competing clothing stores), it's a ubiquitous service with over a billion people in it, almost everybody on the internet.
Like Google, it's more of a basic internet service than a mere website. And its content (and content policies) are a factor in political discourse, both in the US and outside of it.
Secondly, to restrict the discussion to examples that your audience will clearly dislike ("Hitler was right", "white supremacist exhibit") is misleading, because the problem is with items that are not that clear cut but will be censored anyway.
E.g. "Iraq doesn't have WMDs", "CIA is involved in drug trafficking", "US supports death squads in Latin America", "Dodge the Vietnam draft" and so on -- to limit the examples to such items from the past. What would a mainstream company who "censors" stuff allow from those back in the day when they were hot issues?
Or let's take it to today, how about pro/anti-Trump, or pro-anti Assad, or pro-anti Black Lives Matter, pro-anti Manning, pro-anti Assange, etc?
Even stuff that the majority in the US might disagree with, the majority in another culture/country might legitimately agree (and not want it censored) -- but they'd have no say. A single country (and one from which many countries have scars from) will control a large part of the internet discussions (through Facebook, and similar policies in Google, etc) of other countries.
Why it's censored? And, btw, Iraq _did_ had WMDs.
Except in a huge stretch of the notion, that doesn't justify invasion, war, hundreds of thousands dying, and trillion spent -- some degraded barrels of mustard gas and the like from 30+ years ago, the era of Iran-Iraq war...