The classes ui-btn-left and ui-btn-right were not meant to be used in footers, because they do not account for the possible presence of text, navbars, and and other elements often present in footers. You can nevertheless achieve a similar effect when you add a bit of custom CSS.
jQuery Mobile has a feature to automatically create and append "back" buttons to any header, though it is disabled by default. This is primarily useful in chromeless installed applications, such as those running in a native app webview. The framework automatically generates a "back" button on a header when the page plugin's addBackBtn option is true. This can also be set via markup if the header has a data-add-back-btn="true" attribute.