I believe we are trying to be close to Adobe in behavior in certain areas. In Adobe, the eraser tool will still erase path points from a free hand annotation (even when grouped), but it will not delete other annotations.
However, this can be confusing in WebViewer when the eraser has a path indicating what will be deleted. So the eraser tool in WebViewer will delete annotations as well as remove points from paths. Being able to delete annotations when part of a group is likely just an extension of that behavior, as well as treating annotations as drawings on the document.