Many years ago I started work in the field of advertising, I learned a lot about copyright and ownership in it's various forms, and as a quilter (and a musician) I've been most interested in the "personal use" part of it all.
As far as quilting goes, you can see anything you like and make a direct copy for yourself or be inspired by it and make an original copy or turn it into an original design. The problem comes then with what happens next. If you show it, you should credit the originator. As already mentioned some patterns already come with the disclaimer about not to be used for re-sale. Some times that actually means the pattern itself, other times the end project. The typical home sewer does not need to worry so much about that, but yeah -- if you take someone else's work and creativity and then make a business out of it you might just morally owe someone for that even if the legal claim isn't quite as good and often the legal claim is very good.
Because the amount of change required to make something "original" varies. Just using different fabrics could be enough. Slight changes can be enough, or not enough if it is really distinctive. For example, I drew up a Hazel Hedgehog that's slightly different mathematically from the design by Elizabeth Hartman. But I know it is completely based on her design and that's the only reason I did it, so I bought the pattern I figured it was only fair.
Or recently here on the board I helped someone redesign a block. Although I am fully comfortable that the way I resized and redrew the block and wrote the very different directions was an original product, again, the only reason we were doing it was to copy something someone else did and I elected not to share those publicly.
I was at a quilt show yesterday and much to my surprise there was a quilt there that was the same block as one of my current projects. That quilter gave credit to the pattern used, while I know the block as a traditional/free use (no one can copyright a traditional block, we can all make churn dashes if we want!). But her project was very different than mine in colors and placement. If I decide to show my quilt I won't be attributing it to anyone but myself.
I know there have been original works of mine that I have seen later done very similarly by someone else. Great minds think alike after all. I have some projects of mine that are original and I hesitate sometimes to post pictures of them, I don't mind sharing the idea at all -- but I would mind someone taking that and turning it into a commercial pattern.
Copyright/intellectual property does matter to each of us, whether it is pirating or performing music or in quilting. But there are just so many notes and so many ways to fit triangles and squares together
