Also there's a difference between the expectation that the letter can be truncated or just the gist of it extracted for reasons of space, and entire sentences or paragraphs rewritten in full. I would have never expected the latter even with a "right to edit letters reserved" disclaimer.
A letter to the editor might read:
> I saw on your article about the size of foodstuffs that food items are not getting smaller - actually people are just getting bigger. Very interesting article, quite funny and relevant as I've been collecting food items since I was a child and can confirm that this isn't always true. I've got a few exapmles where it holds, but I also have a kit kat from when I was 18 (born 1979) that used to be 500g and now they are 250g as standard. I'm a big fan and this is the first time I have spotted a huge glaring error! Please explain that one!
Clearly that would need to be reframed. I'm not particularly good, but I can see why a newspaper might want to shorten it:
> In last week's article The Economist argued that foodstuffs were not getting smaller, however I own a standard kit-kat from the 1997 that is double the size of ones available in shops today. Thoughts?
The above is probably an improvement from a readers perspective, conveys the points of the original message, and they aren't usually attributed to a specific, identifiable person anyway (usually something like John from London) so I don't think there is any huge scandal here.
(As someone who writes for a living, I can tell you that it takes a little getting used to, especially when an editor butchers a sentence you're pleased with. But it's part of the process you learn to accept eventually — and they were probably right.)
https://youtube.com/watch?v=fkLX58ZWbWw&t=14m20s
(From 14m20s -- 16m30s)
I do occasionally revert changes when I'm reviewing--usually because I wrote something poorly and an editor misinterpreted--but I reject relatively few suggestions. I figure that if I wrote something that confused my editor or they just didn't like it, a lot of readers will feel similarly.
To the extent the comment is atrributed to the author whose words were altered it can be.
An published, attributed comment is a published fact claim about what the author has expressed, and if it is false and damaging can be libel.
The Economist wishes to publish from a wider set of contributors and its house style is harder for that group to imitate.
If you don't like the terms, don't engage.
Edited to add: newspapers in the UK take libel law seriously so you have some confidence they won't grossly misrepresent your views.