Hi,
First link provided is broken. I can't tell anything from the screenshot. If it was a public site I would view the source of the rendered page to see whats going on with the markup and contents.
The forums are configured to force paste as plain text because users often copy some html fragment from a page and paste it in and it causes problems because its missing opening or closing tags or has other invalid markup. So forcing plain text strips out all the markup and is the safest way to allow pasting. The popup only happens in Firefox, not in IE. This is a feature of FCKeditor and how it handles things differently in gecko browsers vs IE.
You'll notice that the toolbar in the editor has item for paste as text as well as paste from word which helps cleanup things copied from word. Unfortunately we can't always count on users knowing enough to use these toolbar items. The Force paste as plain text feature is equivalent to clicking the Paste as Text toolbar item.
Hope it helps,
Joe