Hmm, perhaps instead of "rationale", I should have said "philosophy behind" the 1st Amendment. Obviously the 1st Amendment is not directly intended to cover entities other than the government. But almost everything in the Bill of Rights is essentially intended to protect the rights of minorities against the tyranny of the majority, whose representative is the government. It is not necessary to protect popular views, from either the government or from others.
This philosophy is as old as the university and modern science. It is certainly no younger than the "free speech, but not consequences" argument, which I read as a thinly-veiled way to threaten unpopular views with unspecified "consequences".
> "private businesses can't have moderation"
I wasn't arguing that they can't. Only that it can be unwise and a net negative for our society for them to choose to do so. I would draw a distinction between trolling/spam and someone honestly espousing an unpopular view.
It is true, some kind of filtering is needed, because there's too much information for any individual to absorb. The question is, who (if not me myself) is doing the filtering and what are their incentives? dang's incentives are aligned with mine: good-quality technical discussion, and people aren't blacklisted purely for espousing unpopular views.
The incentives of corporations are not. Google, Facebook, and in this instance, Namecheap, are presumably taking these actions to safeguard their image and maximize profit, not to actually make their users maximally informed. There is a conflict of interest here.
Finally, I think all this becomes much more important when it comes to political speech, because people have a marked tendency to want to shout down, ignore, misrepresent, or otherwise not engage with political views that make them uncomfortable. Look how the media misrepresented the Damore memo. I take no position on the memo itself, but do assert that people are better off knowing actual arguments from all sides, rather than side A blacklisting side B and then giving strawman representations of what B thinks.