I'm less clear on why API's shouldn't be copyrightable. An API isn't just a few lines of code. It's often an intense creative endeavor that ultimately represents a very specific (sometimes innovative) way of looking at the world. Sometimes, those views are uniquely terrible or uniquely elegant... but what's clear is that they are not "natural" in the way that a mathematical equation or physical law may be. It's clearly an act of creation, at least in my mind.
With that in mind... What are the best reasons to deny copyrights to API authors in a legal system that allows writers (who create intense creative endeavors representative of very specific and often innovative ways of looking at the world) copyright protections?
(Mind: I'm not against reforming said legal system; however, given the status quo, I'm not convinced about the differentiation between an API and a written work as creative (and copyrightable) works.)
(Disclaimer: I'm a huge fan of open source and being as "free" as possible. I love Creative Commons and fiercely enjoy the freedoms that FOSS can provide to developers and consumers. I think that API's are something that should be open rather than closed, but I don't think that means that all API's MUST be copyleft / public domain.)