The tldr is: In the fashion industry there is limited protection for innovative designs and as a result, copying is rampant. Despite the ease of copying, innovation is quite strong in the industry and there is a great deal of competition.
I would add that, (1) the clothing industry is similar now to the the time IP was being legislated so it was directly addressed rather than derived by similarity to other areas. Its a little more logical and true to the original goals of IP than areas that derive. (2) There is a big element of hivemind/zeitgeist/emergence in fashion because people invent their own outfit every time they dress. (3) The combination of innovative fashion & quality is what makes high fashion brands valuable. (4) the lack of IP protection actually fuels innovation as a design will remain somewhat exclusive for a season only.
What I find most humorous about those that are so bent out of shape about the copying is that they themselves are copycats, but they hold themselves to different standards. To them, it is okay to copy a design from 10-20 years ago, or to pull 10 different design elements from a current design...but if someone does it to them, they want to sue. The simple fact is that design elements of contemporary fashion are always recycled. Runway fashion is innovative (from an art and design perspective, as much as this crowd would probably laugh at that), but not marketable, as most people (even fashionistas) do not want to be walking art exhibitions.
http://www.ted.com/talks/johanna_blakley_lessons_from_fashio...
They sued Calvin Klein when they started to make some similar pants. It was settled out of court:
http://www.theglobeandmail.com/globe-investor/lululemon-calv...