[Илья Кантор]

Да просто по-разному устроены события там.

if (event.which == null)
       /* IE case */
       button= (event.button < 2) ? "LEFT" :
                 ((event.button == 4) ? "MIDDLE" : "RIGHT");
    else
       /* All others */
       button= (event.which < 2) ? "LEFT" :
                 ((event.which == 2) ? "MIDDLE" : "RIGHT");