WordPress.org

Wy my preferences are reseted when I unistall the plugin?

All the preferences are deleted for provide a best performance of wordpress. So, if you remove this plugin, all your preferences goes away.

My users can write in any categorie. Why?

Administrator have access to all preferences on your blog, this inclues the administration of user-cats-manager, so hano no sense it have restrict access. If the users are setted as Administrator, it can write on any categorie. So, for new users choose editor mode.

You can read more about in http://codex.wordpress.org/RolesandCapabilities

You can use too the Role Manager to have more control of your users. http://www.im-web-gefunden.de/wordpress-plugins/role-manager/