The underlying concept of "Intellectual" + "Property" to me seems steeped in exploitative double-think seeing as how we're wired to adopt incorporeal impressions of culture, societal norms and tools (i.e. language) from the day we're born. A better term for the issue should be "Cognitive Custodianship & Recognition" as we are all inheritors of a global legacy that's been developing long before us and will continue after we've passed on.
As far as reproducing the Artist's original work - the exact artwork, downloadable files, text etc. offered on the website - I can see validation for respecting the artist's wishes regarding creative commons & licensed reproduction.
As far as reproducing the essence of an artist's work when that work stems entirely from the pre-existing commonwealth of human inheritance I see no grounds for justifying any claim to authorial copyright regarding the "intellectual" aspect of the work.
George Bain's book provides sufficient information for recreating and learning how to construct Celtic-style knotwork. Very user-friendly and legally legitimate.