It's long been accepted in book copyright that imitation is the sincerest form of flattery. Plagiarism requires copying of all the details, not mere imitation of the best ones. And it's copyright that should govern software, not patents.
There's also design registration, where there has to be freedom to do things differently before a design can be registered. For example, the external shape of a car exhaust pipe is dictated by the shape of the car it has to attach to. There's no freedom to do it differently, so the design cannot be protected. (This has been tested in the courts). With an Iphone or Tablet there is limited freedom to do things differently (limited, for example, by the shape of a pocket and a human finger). Android is indeed different, where external constraints do not dictate the design.
FWIW I'm so disgusted by Apple's recent antics that the company has joined Sony on my personal boycot list.