They know they cannot rely on a stable API, which means they have to carefully read the diff for every single upgrade, so saving no work over explicitly picking a major version.
By labeling as v1 you're:
- communicating the package is stable and will be maintained
- communication that breaking changes will now increment the major version (saying nothing about stability, backported bug fixes, or any maintenance)
I wish they were not combined. Incrementing major versions frequently on early stage projects would make them work better with package managers, but without the expectation of maintenance from the community.
Not incrementing major versions makes it hard for package managers to make sensible decisions.