Maintenance and security releases, or minor releases for WordPress should be safe, without breaking sites, plugins, or themes, but this year WordPress had some versions that break sites with automatic updates. Since plugins and themes can get same treatment too, some people don’t want this kind of updates, even if their imporve security for previous versions.
While there are plugins that prevent background automatic updates, they don’t do it by default and you need to manually choose what kind of automatic updates you don’t want, I have created new plugin that does this but that works out of the box, without need to select any options, automatically with activation.