Erm... I don't think copying successful business is very risky. You can, of course, bork execution, but the key part - exploring the "needspace" and finding a viable business model - is already done. And even executing, when someone executed before, is rather easy.
As said by others, there are language, cultural and legal differences.
EDIT (missed a piece): Execution is not only design and coding, it's also verifying the market, adapting the product to this market etc..
note: i'm not justifing them (at least not completely), just making some point.
I'm working on creating a better Startup Community end Environment in my surrounding, not only because i'd like that to be here, but also because i'd like to show that there is a another, if not better, way to do this.