Cheerio-pissing take 2: Is it automatically okay because it was posted publicly on the internet?

I posted here the other day that the meta debates going around about squee-harshing or "pissing in someone's cheerios", as well as other widespread opinions and practices in fandom, clearly demonstrate that there is in existence in fandom a norm (or etiquette rule, or expectation, if you prefer) which requires a certain level of polite reserve and social distance. Our community clearly does to a great degree limit the acceptability of stark argumentative replies to someone in their own journal.

I've seen counter-arguments in several posts, most recently in musesfool's[1] such as this and this, which hinge on the commenters' right to comment as they like in a public post. This argument isn't a sensible contribution to the debate, since recent posts (including the post which those comments reply to) have explicitly acknowledged that they do have that right, and focused instead on saying that they should exercise tact in determining whether to use it. When they say that everyone has the right to comment in public posts, what they really seem to be arguing, often, is that it is automatically socially acceptable to comment however one wishes either (a) because the post is public (and the poster has consciously chosen not to make it impossible to comment) or (b) because it is universally automatically socially acceptable to say whatever you want to on the internet.

In my first post linked above I discussed the fact that posts are public and some of the reasons (mainly practical) people make public posts even though they don't readily welcome any and all comments.

Now I want to touch on (b). ( Is it universally and automatically socially acceptable to say whatever you want to on the internet? )