Has that ever actually been tested though? Ultimately, to work, a plugin only needs to call a handful of WP functions, if at all -- hard to argue it's a derivative work, just because it can be used _with_ WP. Is grep a derivative work of bash because you can use bash to execute grep?
Something like the following would be a valid plugin, but it doesn't directly or indirectly use _any_ code by WP. It's only executed by WP. I don't believe that it would be considered a derivative work of WP, because that file would happily run without WP as well.
<?php
/*
Plugin Name: My Plugin
Version: 0.1
*/
if($_SERVER["REQUEST_URI"] === "/my-url") {
// do something interesting, use curl to fetch some content
}
I've always been confused by this assertion, too. It's often the justification for privacy of paid plugins, too.
In addition to your example, I can imagine a very defensible scenario where a plugin is "standalone" with a shim layer to connect to WordPress (and maybe other CMSes, too).
Yeah, the infectious nature can hardly infect a third party software, even if the shim itself would be infected.
The one case I just remembered where it was remotely challenged was Thesis [1], but Automattic chose to throw money into bullying the challenger rather than going after the alleged license violation.
Something like the following would be a valid plugin, but it doesn't directly or indirectly use _any_ code by WP. It's only executed by WP. I don't believe that it would be considered a derivative work of WP, because that file would happily run without WP as well.
<?php /* Plugin Name: My Plugin Version: 0.1 */
if($_SERVER["REQUEST_URI"] === "/my-url") { // do something interesting, use curl to fetch some content }