Copyright has a legal basis. It gives the original creator of a piece of work the exclusive rights to that work. Normally such rights are given for a pre-determined period of time. It infers the right to copy and gives the originator the right to be credited for the work and to then allow others to use and/or adapt that work. It allows the originator to determine who can gain financially from the work and it forms a key component of intellectual property rights.
A derivative work includes copyrighted elements of a previous underlying piece of work. In order to be considered a unique piece of work from the original it must bear the stamp of its creator, the transformation must be substantial. Derivation is only protectable when it bears original expression, it’s derived elements are non-protectable.
Fair use of original material needs consideration, rights and privileges of the individual in society to privacy, and as equitably as possible the right to freedom of expression lead to intrepretations of copyright law.
Copyleft adaptations of copyright laws are pieces of work that can are by their very nature are ‘protectable’ from individual copyright law. Copyleft helps to insure that any future use of a work that has been copylefted must also be copylefted.