I would leave the issue of following the copyright stipulations up to the quilter. I have never bought a quilt pattern. They are easy enough to look at and duplicate on my own. So I may have gleaned lots of ideas from quilts I have seen, but they are nobody's exact "pattern". The quilt you saw at a show may have been made the same way-- by inspiration, rather than pattern.
I agree that the Tabberone site is not reliable or lawsuit-proof. Plus, the "patterns" they are referring to are clothing patterns, which involve templates, rather than artwork or written words. People on knitting sites quote Tabberone all the time, to get around copyright on knitting patterns, which are written words, rather than shape templates, as the Tabberone article is talking about. Apples and Oranges...