Using a similar idea is one thing, but cloning is another. They don't just take the concepts, they rip of the design as well. If you look at the Samwer's version of Fab (which is shut down now), they completely ripped off the look and feel, you can see a screen cap and read this article http://www.businessweek.com/articles/2012-02-29/the-germany-...
Because suing someone who operates--and serves customers who are--entirely outside of your legal jurisdiction is extremely difficult, expensive, and still has a low probability of success even if you're in the right. This was basically the reasoning behind SOPA's "let's just take their domain away if we can't sue them" provision.
Imagine for a second if Samsung only sold phones in Korea--do you think Apple would have bothered with a suit?
