There are some editors, generally (but not exclusively) these are younger editors who learned to write on a keyboard, and detest the ancient typewriter art of adding two spaces after every period (aka full stop).
Let me point out that people who learned to type on a typewriter and not a keyboard had this habit ingrained into their process. My thumb automatically adds two spaces after the symbol that ends this sentence. Notice that two spaces appear.
When I’m writing, I create at as much as 1500 words an hour. This is not the legendary speed of Jay Lake, but it’s a lot faster than most fiction writers. When I’m writing non-fiction and just relaying facts I know well, I can type even faster.
Here’s the thing. If you want me to change the layout, that causes me to think about something other than content. That slows me down, and I guarantee I will have two spaces after several of the periods regardless. It’s ingrained. It’s ingrained because I learned good typing techniques on a typewriter.
See, this is a formatting issue, not a process issue. I recently heard of an editor who will reject a manuscript on the basis of how many spaces appear after a period–without even looking at the content.
Here’s my proposal. You don’t like the way I type, fine. I will alter the formatting upon acceptance. If that isn’t good enough, then I don’t really want to work with you.