There are multiple factors at play. It could be due to the original poster's preferences in terms of sharing. They might not want their tagged story to be spread around. Also, the platform might want to ensure that stories are shared in an appropriate and legal manner. Reposting a tagged story without permission could potentially violate copyright or privacy laws, so it's blocked.
Well, there could be several reasons. It could be that the owner of the story has disabled the repost option for privacy or other reasons. Or maybe your account doesn't have the necessary permissions. Another possibility is that the platform has certain technical glitches at the moment.
'Can't repost tagged story' means that the system or the rules don't allow you to share a particular story which has been tagged. This can be to safeguard the intellectual property of the person who created the story. Also, in some social media platforms, if a story is tagged for a specific group or purpose, reposting might violate the intended use of that tag, so it's prohibited.
In most cases, no. Reposting without being tagged might violate the original poster's expectations or privacy. It's better to respect their intentions.
Generally, you shouldn't. It could be seen as inappropriate or even against the terms of service of the platform where the story is posted. Plus, it's respectful to only repost when given explicit permission or when tagged.