Why does b flat trombone not transpose on reading bass clef?

B-flat trombone transposes when playing in bass clef. When a B-flat trombonist reads a note written in bass clef, they play the note that sounds a whole tone higher. This is because the B-flat trombone is pitched in B-flat, which is a whole tone lower than the C that is written in bass clef. For example, if a B-flat trombonist sees a whole note C written in bass clef, they will play a whole note D-flat.