I'd say that all artists draw from created content in one way or another, whether it be watching, listening, experiencing some form of art and taking that as inspiration, or literally using that created art in the art that you're trying to make. Artists are constantly looking for inspiration because otherwise, where would their creativity come from? For example, in the documentary, Girl Talk walks us through his creative process which involves borrowing already created creative content. He describes it as a new genre of music rather than an attempt to sabotage the labels. The majority of these creators are simply trying to create their own creative content by borrowing from previously creative content. Almost as though the process they're using is the creative part, not the content itself. I believe that when it comes to mixing, borrowing, etc., artists should have the freedom to do so.
I believe that when borrowing is transformative rather than derivative, it should be okay, as long as the person gives credit to the creator. However, when someone is simply being derivative, I believe that it becomes a grey area. The artist should make a solid judgment call on what they're doing. I wouldn't say that it's totally wrong to be derivative but it's certainly not totally right. Neither is being transformative. I think determining the right/wrong means looking at each situation separately. In terms of who's looking at each situation and then determining whether it's right/wrong, I'm not sure. It might need to be based on what the users are saying.
In terms of music that uses sample, I've listened to many Hip-Hop songs that have sampled things. One artist I enjoy who samples other songs would be Kanye West. I'd say the vast majority of his songs have sample at least one other song. In terms of movies, sampling would more so be used in "found-footage" esque, indepdent online videos, rather than feature length films. However, if a film needs news footage, old-timey footage, etc. then they would obviously need to credit the source in the film. Both of these examples have their grey areas but for the most part are safe.
If, for example, another artist used my own created content in one of their pieces. At first, I'd probably be flattered. Then, I'd investigate the content and make sure that I'm getting credited. If I am credited and my content is being used in a "creative" way in order to enhance the content that the artist is creating, then I'd be content with that artist using my stuff. However, if I'm not credited and/or my content isn't being used properly, then there'd be a problem. So, based on that, I'd say that if an artist's work has been used by another artist in some fashion, it should be the artist's call on whether or not their art is being represented and distributed properly.
On the flip side, I've had to use already created content before, in a way that uses that content creatively for my own purpose rather than trying to simply distribute the artist's work unjustifiably. I'd say that as long as you give the artist credit and are doing your own thing rather than there's, it should be okay.