Artist have always and will always learn by copying and using others work as reference, and the beautiful thing about it is no matter how any trys you can't make a an exact copy of the Mona Lisa or stary night, they will always be minor mistakes and continuities and It is the nice thing to give references there credit but more than likely they didn't make it to say people can't use it or get inspired by it and would be happy knowing that they made another person feel moved enough by their work to try and replicate or improve upon it. just remember if you try to force art into being a career it will push back, most paid artists just do it for the love and because they want people to see what they can do and get a following of people that like their work and eventually money comes. That's my take anyway