> However, just because FSF thinks linking is enough to make derivative work doesn't mean it necessarily is

Exactly. In the EU "linking two programs does not produce a single derivative of both (each program stay covered by its primary licence)" because Directive 2009/24/EC allows this: "any portion of code that is strictly necessary for implementing interoperability can be reproduced without copyright infringement." https://joinup.ec.europa.eu/collection/eupl/matrix-eupl-comp...

On the other hand if you view the license as a contract, I believe it can make arbitrary demands about the license of other (non derivative) software.

edit: actually even if you consider it a contract it probably can't prevent you from doing something that is allowed by the law, so it can't affect the license of other software. I'm not a lawyer.