According to the Amazon SimpleDB Developer Guide : "To ensure that you can read all the data you sent via REST, if a response contains invalid XML characters, Amazon SimpleDB automatically Base64-encodes the UTF-8 octets of the text. When a returned element is Base64-encoded, its encoding element is set to base64. The following example shows Base64-encoded results from a GetAttributes operation."
Base64-encoded are not decoded in _parse_attributes method.