Peckish

It is not illegal to copy an idea. It is illegal to copy someone else's written instructions and pictures, and sell them as your own. Looking at a picture of a quilt on Pinterest and figuring out on your own how to make it is not illegal.

The reason why so many people take "public domain" and traditional quilt designs, write up instructions, and sell the pattern is because there are always buyers. New quilters come on the scene all the time. Some people like to have things all figured out for them already, so they don't have to do the math. New techniques and tools are developed which make the pattern easier, and we need instructions on how to use them.