Well, from the end-user's perspective they care very much that the clone can provide the same functions organised in the same way as the original, because otherwise their scripts that run on the original interpreter won't run on the clone.

But yes, the underlying implementation is still subject to copyright (and potentially, patent). You can't copyright the fact that your scripting language has an operator spelled >< that concatenates strings, but you can copyright your particular code that performs string concatenation.