I have been seeing new users who copy/paste code or information into posts not add extra newlines(or even type without extra newlines) and make posts hard to read, wasting the time of a moderator or editor. Can this behavior be changed to make single linebreaks act as lienbreaks? Or is by design?
It is by-design - the Markdown syntax specifies a blank line between paragraphs.
Code should be indicated as such - indented four spaces. There's a button on the toolbar and a shortcut key to accomplish this, and doing so also handles escaping special characters in cases where it otherwise can't be done automatically.