Shack isn't a usual term for store, so it's less generic. Allowing generic words to be trademarked too easily causes problems. International Business Machines

Shack isn't a usual term for store, so it's less generic. Allowing generic words to be trademarked too easily causes problems. International Business Machines seems like a fine name right up until there are many multi-nationals making business machines. At that point IBM starts to sounds more like a descriptive (It's an IBM, it's got English and German on the keys...) Selectric on the other hand, seems like a name the public benefits by protecting.