Because the editing software is prehistoric.

Complicated reasons. Talk pages do get moderated in specific situations, and those do not require special permission. For example, anyone can and are encouraged to move old discussions into archives.

Additionally there is spam. Spam need to be detected and removed from the talk space and administrators can't be the only one with permission to do that.

That said, there is no excuse that the UI don't inform you about the current norms that exist.

> If there are areas where contributions are discouraged

Editing someone else's comment is vandalism, not contribution.

I beg to differ: I did not vandalise another user’s comment. (But then again, I would say that, wouldn’t I, since I am arguing my case and I am inherently biased in my favour.) I will provide the facts so that you can assess the facts for yourself:

I didn’t just change his spelling. I edited his spelling, but I left the previous state (in another section), explained what I had done and why, provided sources and citations as to why my transliteration was preferable, and took clear ownership of what I had done and why I had done it.

What you did was not vandalism, but it was rude, since it violated the moral right of the other user to control the words associated with their authorship. When one contributes to the article space, one is explicitly relinquishing that right, because Wikipedia is a collective effort, but that does not hold for talk space content, particularly when it is signed.

And what you did was doubly rude because you acknowledge that your version was "preferable"; that is, it is conceivable that the other user prefers their version. They may be wrong to do so, but by editing their words directly you are denying them their right to an opinion. It is not surprising at all that your edit was reverted.

Yes, I agree, in hindsight, that I was rude according to what I discovered (the hard way) be the prevailing norm.

However I have difficulty agreeing with you on your second paragraph, indeed it is the exact opposite of what I deeply believe: I am convinced that there is an objective truth, and that one of the things where there is prevailing conviction as to what is correct spelling wand what is not. Words have meaning by virtue of convention, and if you do not pursue the convention, you are reneging on the certainty of being able to unambiguously and intelligibly commune with others.

Words change over time. One of the big reasons is being prone to misspelling.