This has impact beyond APIs. The Supreme Court says copying User Interface is fair use.
Excerpts:
The nature of the work at issue favors fair use. The copied lines of code are part of a “user interface” that provides a way for programmers to access prewritten computer code through the use of simple commands. As a result, this code is different from many other types of code, such as the code that actually instructs the computer to execute a task. As part of an interface, the copied lines are inherently bound together with uncopyrightable ideas (the overall organization of the API) and the creation of new creative expression (the code independently written by Google). Unlike many other computer programs, the value of the copied lines is in significant part derived from the investment of users (here computer programmers) who have learned the API’s system. Given these differences, application of fair use here is unlikely to undermine the general copyright protection that Congress provided for computer programs.
You're reading too much into that. The user in this case is the programmer using the API, or even the code that uses the API. It's doesn't generalize to end user interfaces.
Excerpts:
The nature of the work at issue favors fair use. The copied lines of code are part of a “user interface” that provides a way for programmers to access prewritten computer code through the use of simple commands. As a result, this code is different from many other types of code, such as the code that actually instructs the computer to execute a task. As part of an interface, the copied lines are inherently bound together with uncopyrightable ideas (the overall organization of the API) and the creation of new creative expression (the code independently written by Google). Unlike many other computer programs, the value of the copied lines is in significant part derived from the investment of users (here computer programmers) who have learned the API’s system. Given these differences, application of fair use here is unlikely to undermine the general copyright protection that Congress provided for computer programs.