How the GPL works, or not

I have to say that at one point I really liked the GPL license and the idea behind it. I always thought that it is weird to call it viral.

According to the interpretation of the Software Freedom Law Center the GPL might be viral after all.

Consider their statement on WordPress themes:

The PHP elements, taken together, are clearly derivative of WordPress code. The template is loaded via the include() function. Its contents are combined with the WordPress code in memory to be processed by PHP along with (and completely indistinguishable from) the rest of WordPress. The PHP code consists largely of calls to WordPress functions and sparse, minimal logic to control which WordPress functions are accessed and how many times they will be called. They are derivative of WordPress because every part of them is determined by the content of the WordPress functions they call. As works of authorship, they are designed only to be combined with WordPress into a larger work.

I’m not a lawyer, but if using a proprietary API makes something a derivative work, wouldn’t that mean that something written to a proprietary API covered by the GPL also has to be redistributed under the terms of the GPL? At what point wouldn’t something be a derivative work anymore? Is there even a point where it’s no longer considered a derivative work?

Do I even want to bother with this when writing code or should I just choose to work with a license that I can actually understand without a team of lawyers?