Inspiration or Plagiarism?

Designers seem to be taking more and more inspiration from artists these days. But how much inspiration can be counted as plagiarism? Designers from YSL, Dior, D&G, Gucci, Rodarte and even high street brand Free People are taking what they call "inspiration" from the most famous valuable paintings in the world. 

Even though I believe D&G, Gucci and Free People used these artists as purely a starting point to their designs I would be interested to know how the artists would have reacted. YSL, Dior and Rodarte however, copied their “artist inspiration” entire pattern and simply placed it on a dress I don't see how this is right? These artists they are coping spent years on these paintings to get their composition and colouring right and for a designer to mimic it and place it on a garment and say look what I've created seems selfish and wrong. 

 This is just my opinion so please tell me what you think?