In the release editor, there is no indication that annotations are released under a CC license and therefore you shouldn't copy and paste big chunks of non-factual text:
On https://musicbrainz.org/release/add the bubble says
The annotation field functions like a miniature wiki.
The purpose of this field is to add information that usually doesn't fit into the strict structural data schema of MusicBrainz (be it due to technical limitations that may be addressed later, or because the information in itself has to be free-text).
whereas on https://musicbrainz.org/release/b7b26c25-78d3-4243-a181-62e8dc97dadd/edit_annotation the text above the textarea says
Please note that any content submitted to MusicBrainz will be made available to the public under open licenses, do not submit any copyrighted text here!
I doubt that changing it will actually stop people, but it's hard to tell people off for doing something we didn't tell them they shouldn't do.