Saint Louis is the unabbreviated form of St. Louis, not a misspelling.  There are many local institutions and businesses that utilize Saint Louis without it being considered a misspelling.  These include the Saint Louis Public Schools, Saint Louis Zoo, Saint Louis Science Center, Saint Louis Art Museum, Fair Saint Louis, Saint Louis University, among others.  At least three neighborhood associations use Saint Louis: Saint Louis Hills, Old North Saint Louis, and Soulard Saint Louis. The USPS uses Saint Louis as well.  I can share more examples if you need them.