Perhaps they are a safe choice because the group members are either weird or are otherwise quite independent and reliable. In fact most of them are monsters with no connection to the civilization. Aria and Vulcan are a different thing, but they still aren't tightly tied to any larger entity, like a royal family, by being knights or anything of that sort. They are honourable and likely to keep a secret if asked. As adventurers, nobody will pay extra attention to their comings and goings. While many would grieve for their deaths, it's still something that often happens to adventurers, unexpectedly. Furthermore, they possess power, making them worth it.
