Indeed. It is a signal for humans, not for the package manager itself.
"Proper" semantic versioning would not bring any benefits, because when two packages both depend on an unstable package, they would have to agree on a version to use. When the major version is bumped constantly – because the goal of the package is to break source compatibility constantly (if only to add new API, which is technically source breaking) – then this would cause a logjam by giving the illusion of being useable by source-stable packages without actually being so in practice.
The fact is this package should not be pulled in by other packages that want to remain source stable. It would be nice to find a solution for these use cases too, but I've yet to see one, and holding up the preview package while we search for one, preventing apps and their supporting frameworks that could benefit from it. Once we do resolve this issue I hope we can adapt the process to use it.