The reason that the "DO NOT PASTE!" message is displayed so prominently is that many people will copy and paste from a word processing application like Microsoft Word.
These applications typically use "smart quotes" or "smart apostrophes" that create characters in messages which carriers reject. Only standard quotes or apostrophes are allowed.
We have built a detection mechanism into the text creation box that sees any of these illegal characters and shows an alert box when you attempt to send the message. While we are confident of having addressed as many of these illegal characters as possible there may be some we have missed so rather than take the chance on having the error message appear and confuse our users we advise people not to paste text at all and to simply type the message into the box directly.
That said, if you are using a plain text editor that does not have any styles, different fonts or other non-text attributes, you should be safe to paste text in from them. If there's a problem with any of it the error message will appear.
That's why the alert message says "DO NOT PASTE!"