I think that is because of the (possible) impact on the applications logic and complexity when having multiple culture versions and urls.
As an editor you can edit the url by updating the URL Slug on the URLs tab. If you have enabled the 'Store former URLs:' in the setting this will automatically create a forward (redirect or rewrite based on your config) for the "old URL". I think that's also better to not clutter alternative urls with typos and page moves.
You might be able to get the logic to the Content Tab, check "CMSModules/Content/CMSDesk/Properties/Urls.aspx" to see how Kentico handles it...