Not only it injects newlines, it injects indentation. You can always 1) paste from logs 2) paste from the in-memory rawlog Irssi keeps or 3) use a script like xsel | tr '\t' ' ' | tr '\n' ' ' | sed 's/ */ /g' | tee >(xsel -i) | xsel -ib to your clipboard (yes it uses tr uselessly)

Copy-pasteability is not really the goal of apps that use ncurses or similar interfaces. Side-by-side windows break it also.