The point is that if you copy something, it's still someone else's work. You've just made a lifeless derivative. If you steal it, however, that implies more than making a copy - it implies making it your own. Things stolen no longer belong to the original owner.
An example: House of the rising sun is a traditional song, passing round loads of musicians - but the Animals "stole" it in such a way no-one can hear it anymore and not think of their work.
Good artists copy, great artists steal.
The point is that if you copy something, it's still someone else's work. You've just made a lifeless derivative. If you steal it, however, that implies more than making a copy - it implies making it your own. Things stolen no longer belong to the original owner.
An example: House of the rising sun is a traditional song, passing round loads of musicians - but the Animals "stole" it in such a way no-one can hear it anymore and not think of their work.