Since many make their living writing, plagiarism and copyrights laws protect the fruits of their labors. Taking someone else's ideas without giving them due credit is lying, and it is so easy to attribute to the orginator and avoid plagiarism. Copyright laws are similiar to patent protection. If everyone produced an exact replica of the I-Phone, would Apple consider it okay?
I value lying as an extremely despicable act. Those who knew better and ignored doing the right thing violate trust that are detrimental to the fabric of society. Is it that difficult to give proper credit when it is due?