Why isn't the Byte Order Mark emitted from UTF8Encoding.GetBytes?

UTF8Encoding enc = new UTF8Encoding(true/*include Byte Order Mark*/);
byte[] data = enc.GetBytes("a");
// data has length 1.
// I expected the BOM to be included. What's up?

UTF8Encoding enc = new UTF8Encoding(true);
byte[] data = enc.GetBytes("a");
byte[] combo = enc.GetPreamble().Concat(data).ToArray();

byte[] preamble = enc.GetPreamble();