Just because you believe it to be the case that users of libraries are not derivatives of those libraries does not make it true. Nor does the belief of the FSF and others that it does make that true."
If this were the truth, anything compiled using the GNU compiler would be also under the GNU (since you would be compiling libraries from this compiler straight into your application/binary and calling function names).
It really boils down to the definition of a "derivative work". As an example: A Paypal plugin for Wordpress should not fall under this, yet the developers for Wordpress think it does.
The entire situation with Thesis will only hurt Wordpress in the future. As a developer, why would I want to create any Wordpress plugins or themes if someone else is going to lay claim to my work (which really is my work. If the guy from thesis was actually re-distributing Wordpress along with his theme, it would be a different story)?
It's scary to think that the people that have talked about Freedom for so long are getting just as restrictive and bad as the RIAA/MPAA.