Thesis integrates with Wordpress, it is not derivative from it. Same way Linux modules integrate with the Linux kernel (and many of them are closed source).
If someone is using a public/open API, and not distributing any part of the software, it means they are not based on the other.
See for instance: http://www.oreillynet.com/linux/blog/2006/08/why_binaryonly_...
At what point does dynamic linking make something a derived work - and how is dynamic linking fundamentally different, than, say, a remote procedure call (or SOAP or REST or whatever) over a network? The interface is different, but the principle is the same.
Also - the GPL does state that it's okay to link against things that are "standard parts" of the target system....
http://drewblas.com/2010/07/15/an-analysis-of-gpled-code-in-...