There are kind of two ends to this question, in my observation. There is the maker who wants to use a pattern or design, but make changes enough so that they can call their work original. And there is the designer who wants to make a pattern and hold ownership of that pattern and what might be considered offshoots of that design. And then there are a whole lot of sub-categories of issues under those two categories.
I've seen cases where a designer/pattern maker is miffed that someone makes a design, makes a ton of changes and adaptations, and then still cites them in the end result. It is kind of the equivalent of the people on Epicurious who give a recipe 5 stars, but then in the review say that they changed almost every single thing about the recipe.
And then there are the designers who make up a pattern straight out of their own head, with no outside influences, and they go to market their design and are outraged when people inform them that what they have there is a traditional Irish chain.
The nature of quilting as a craft and as an art is that it has history, it's collaborative, and it's flexible and constantly changing even as it draws on traditions. I have the most respect and appreciation for quilters (at any level of skill or artistry) who recognize this and who give credit for influences and input when that's due, but who are also humble enough to admit that there really is nothing new under the sun, and even the most amazing innovations are drawing on the works of other quilters.
I almost never buy or use a pattern, but I love looking at library books and photos and websites and quilt bloggers. Obviously all of the quilts I look at funnel into my brain and influence what I make up on my own. I can't give credit specifically to the influence, because not one maker or designer is the inspiration to the best of my knowledge. But on the flip side, when people ask me how to do something or how I made a quilt, I'll share my own process freely. For me, that's the beauty of quilting -- free give and take. Admittedly, I do not approach quilting as a potential source of income-- and I know that colors the issue for many. Even so, I think there are good examples of quilters who are generous in sharing their expertise and patterns and still make it as professionals. An example would be Leah Day -- free information on her blog and videos, yet she was voted teacher of the year and has a business that apparently is financially successful.