Some do find them to be offensive because they interpret them in a different way.
I only know that the puzzle pieces and colours represent the diversity of the spectrum and also symbolise how the many different autistic people, their families and friends are affected in their daily lives and connected by autism spectrum disorders. (And puzzles due to the fact that ASDs are still puzzling people, people don't know everything about autism yet, many don't know/don't understand autism, it's hard for them to understand autistic people etc.)
So, now I do know that some people find the ribbon to be offensive, because they say they understand that the puzzle pieces say they're a puzzle due to their autism. So people say that they're a person and not a puzzle of course.
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Autism + ADHD
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The trouble with having an open mind, of course, is that people will insist on coming along and trying to put things in it. Terry Pratchett