Whoever owns or writes the code has copyright on it, from interfaces down to implementation. Anyone who has worked in the software industry for any length of time knows this. There have been dozens upon dozens of lawsuits over the years about people copying functions for spreadsheets, APIs for libraries, and a whole host of related issues. And it always came down in favour of the original author of the interfaces and implementations.
Think about it: What protection does your non-open-source software have if anyone can just use your API to implement an open source variant on the product? There is no way in hell that the entire software industry is going to give up that protection for their products and works.
Some in this thread are going on about the GPL and Linux header files. That only affects people writing drivers -- regular user space code is compiled against the libc interfaces, which are specifically LGPL to allow you to use them for writing products. Products like NVidia's closed-source drivers have always been a legal grey area just begging for a lawsuit to resolve once and for all whether it is permissible for them to write closed source against GPL interfaces. Personally I think if the courts had to make a decision on it, NVidia would lose for the same reasons the commercial vendors want to protect their works. NVidia does not get to dictate what constitutes "fair use" of an API, and neither does Google.
The owner of the API gets to determine what they're going to consider "fair use" and what they're going to consider to be an actionable abuse of their property.
I have never seen any reason nor excuse for the existence of Dalvik and the entire Google stack. Surely the "bright people" at Google could have written some sort of adapter layer for Swing as required for using the OpenJDK/GPL version of Java. But they didn't. Instead they're trying to lock people into the competing Android GUI stack, and are rightfully getting spanked for trying to break the "write once, run anywhere" philosophy of Java.
Google does evil all the time. Sure, Oracle does evil, too, but in this case, it is Google that is the greater of two evils.