Because somewhere between the Preview and Post the board decides that a bunch of non-alphanumeric characters are invalid all of a sudden. (apostrophes, quotes, registered trademark symbol, etc.)
I actually did not use the preview button I just posted it. I thought it could handle a line of text with no quotation marks. Clearly I was mistaken. I had to type the entire line out for it to post correctly.
I am not sure what could be going wrong, I attempted to reproduce your problem four times by copying text from wordpad and notepad into a post with and without formatting and produced perfect quotes.
I attempted once by changing to a font that would not be on this board or website by default, and then copying it and pasting it into the post window. The text reproduced itself successfully without the font style, of course. I also tried it with both using the preview button before hand and without, and also added punctuation in my texts in order to best reproduce the issue to no avail. I made these documents on my computer, typed into them myself, and saved the documents, and then closed and reopened them before attempting any of this.
If nothing else, notepad is about as basic as you can get (ANSI worked for me), if you're having trouble with that I can't imagine what's going wrong without seeing it occur and diagnosing it first-hand. The most I can suspect is some shape or form of formatting or font you might be using that treats these characters oddly, and the attempted reproduction failing as a result.
Here is a link to my test:
http://lucidmagic.net/index.php/topic,3499.0.html