Seriously, tech readers really have to take 5 minutes and learn about trademark law.
That is an industrial design - which is part of trademark (and copyright) law. Under trademark law, a company that owns a trademark has a legal obligation to defend their trademarks. If they don't they can (and do) lose ownership of said trademark. That's why Xerox worked so hard to get people to stop calling photocopying "xeroxing" and Proctor-Gamble worked to get us to stop referring to tissues as "Kleenex".
If Apple didn't issue a C&D, they would be ceding permission for anyone to use that design, which could end up with them, also losing the "iMac" name and all sorts of other knock-on effects.