Pretty sure they said it's because it doesn't support permissions, so you could see anything on your hard drive, that's why they don't support it and do indexing instead
The operating system doesn't need permissions to see anything on your hard drive and it is the operating system that is enforcing the permissions so it can filter the results afterwards. Also, an index can be stale and show files whose permissions have changed that you shouldn't be able to see so the permissions have to be checked on the search results either way.