This is a change in policy from previous Drupal releases. New code and API can be and have been released in minor releases. The API pages are necessary so that it is possible to lookup specific API in minor releases since a minor release may still be valid.
Drupal 8 introduces a six-month minor release cycle. Minor releases provide new improvements and functionality without breaking backward compatibility (BC) for public APIs. The following types of changes are allowed for minor releases in addition to those allowed for patch releases. For example, following the release of 8.1.0, these changes will not be committed to 8.1.x, but instead can be committed to 8.2.x for inclusion in 8.2.0.
Minor releases (8.x.0) may include non-API-breaking refactoring, new features, or enhancements of existing features. In such cases the core team will work to ensure that these enhancements do not alter the existing public-facing API of core systems.
Necessary security hardening takes priority over API stability.
We will make every effort to address security issues without affecting the public API. However, in some cases it will not be possible to address a security vulnerability without an API change. In such cases we will work to minimize the scope of the API change and document it thoroughly.