Surely it's quite the opposite though; the API can *only* be expressed in one form,
That's a tautological argument. The process(es) which the API expresses could be expressed in another form as a different API.
a precise set of class, field & method signatures.
And documented behaviour
! Without that the API is pretty much worthless. With that, in particular in the specification of behaviour between components, you start to get the expression of the idea (IMO).
@Abuse's point is interesting. If copying the entire api is not possible, then compatibility is broken. Applying this logic to vmware and other hypervisors, which rip off entire operating systems' apis, they are all illegally copying (and distributing, @princec) and owe licensing fees to Apple and Microsoft.
Indeed, Java itself rips off windows apis such as the file system, or windowing toolkit, and every other api that exposes operating system functionality. Therefore oracle owes licensing fees to the operating system developers.
Obviously this is silly, but this chain of thought seems consistent with oracle's and some of your arguments.