Big US companies, and all US companies, big and small, have always choosen, and very carefully, what sorts of content they want to distribute, what sorts of image they want to portray, what sorts of causes they want to publicly support, and what their public imagine will be. You were never able to find atheist content in a Christian bookstore, you could only buy the censored version of CDs from Walmart, you couldn't find pornography at K-Mart, The Disney Channel never broadcasted any politically incorrect material, and you couldn't buy t-shirts with "inflammatory religious or supremacist content" in Old Navy. Even the original Geocities had strong content restrictions.
Are you loudly complaining Old Navy doesn't sell a "Hitler was right" shirt? Are you complaining about the "censorship" going on at the Museum of Fine Arts since they don't have a white supremacist exhibit?
In fact, pornography is legal and YouTube does not allow pornography. Why aren't you already up in arms about that "censorship."
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...