I think there is a level of decency involved. If you plainly 'use' files that were written by others, learned from it, modified (a bit), or whatever, and build your own image; no problem, I guess. If you build your scene with many models and other people's files and didn't have much to change (and have a fair chance to become rich), it's good to ask the file/object producers to ask.
I also think people must realize that everything they put on the web, can be used in whatever way and they'll never know. If someone buys my shore foam thing, produces an image, and sells it for big bucks, it is not for me to oppose. I personally would feel a little bad when using someone else's idea/object in a commercial thing, so I prefer to make my own objects/shaders. Although I sometimes have some insignificant things (in the distance) that are made by others, but they'll never constitute the main part of a commercial product.
---Dune