I misread your "I never understand this argument." as not understanding what consp wrote, "this" was a bit ambiguous, and I originally thought you meant "I'm the scientist, let me do science instead of communicating clearly what I did and observed" but now I see "this" referred to the argument consp quoted. So we are both agreeing with him. Add to that that I was confusing a sibling comment with what consp wrote. :)
It's not the cost of a value-adding service, but the cost of having a huge technical debt that nobody has bothered paying down on.
That's exactly what's happening with Open Access Journals: they charge the authors to finance editing and other editorial processes, and distribute papers for free and with liberal licenses.
Why exclude those fields? I would argue that poor editing skills are fairly widespread, regardless of domain.
Copyediting is still badly needed, but that's typically foisted upon a grad student; the journals don't contribute much as far as I can tell.
At least one publisher, despite claiming to 'accept LaTeX', appears to just copy-and-paste from the pdf output into Adobe InDesign and manages to introduce tonnes of new errors, which then I have to have substantial time to fix (for free, again). [And usually I don't manage to catch everything.]
Actually some conferences come with "publishing fees" in the form of "if you want to publish at our conf of course you'll have to present here, which means you'll buy a ticket, which is veeeeery expensive".