In the Wizarding World blood status is an important factor in society. A civil war was fought under the guise of blood purity. In some cases entire bloodlines came to an end. See the first chapter for full summary
Why would you bother to keep someone in protective custody without an able keeper? Mrs Figg was a member of the Order of the Phoenix and could have been a formidable Auror watching over Harry Potter. What if she decided to intervene?