There is a common belief that a software license is unable extend requirements related to derivatives past some boundary because of some externally imposed definition of derivative. In cases where this reasoning is invoked it is usually wrong.
When a copyleft license says "Thou shall not froboz this package with software under a different license" it doesn't much matter that US law may not consider frobozing to be an action which causes the target to be a derivative work: Your ability to receive the rights granted by the license was conditional on following the rules and without those rights you're not able to do much involving that package.
This does potentially leave open the door for vendors of proprietary code intended to be linked into free software, but whom don't actually distribute the free software themselves. Unless it can be shown that the free software's license was required (i.e. if the module were a derivative work) only downstream distributors (or, potentially, users) who need the rights granted by the free license be in violation.
But in the context of copyleft licenses and linking-like activities the person doing the linking-like activity will almost always be distributing the copyleft license covered software. In these cases the license is pretty much free to define its own linking requirements and have them be completely enforceable, since their distribution of the covered software would be a copyright infringement without the rights granted by the license.
The flow chart is:
1) Did you distribute, modify, publicly perform, or otherwise execute a reserved right with the the covered work:
Yes) You must do whatever the license requires. Stop.
No) Continue on to (2)
2) Would a court consider what you distributed to be a derivative work?
Yes) You must do whatever the license requires and you should have answered Yes to the above, creating a derivative is a reserved right. Idiot. Stop.
No) Do whatever you want. Stop.
The misunderstanding comes from skipping straight to (2) without first considering (1).