I don't understand this attitude they are "washed", "verified", etc. My bitcoins are accepted anywhere bitcoins are accepted. Aren't they worth just the same?
Bitcoin is fungable however people often do not wish for the bitcoin in their wallet to be associated with other addresses associated with illegal activity
I doubt most people would know however. When I buy BTC, all I see is the amount that I want and an address (or addresses) with random characters that sends it to me. Same goes for everyone else. Most people (shops included) aren't going to put in the time and effort to trace back any of the transactions that they receive. Plus the practice of using change addresses would make it rather difficult to know whose money actually belongs to whom.