right to copy or copyright

If i knew Isserman I will like to ask him, When do we have to qoute the original source? I understand that copyright is a little soft on academic usage but does it give us the right to copy? Isserman claims that if a statement is repeated over time it become very common that we do not need to name the source. If we keep leaving the source unknown all the time will finally lost the original speaker. Example is “I have a Dream,” if these words keep flowing around with Martin Luther King’s name we will one day foget about the importance of such speech. Furthermore, the digital age has made it more easier for copy and paste that plagiarism is most common among students than ever. Digital remix has all the worse being the way to destruction of original work of arts. As long as we have this tools distributed to everyone we could only do damage control on copyright because students today have the right to copy.