My impression is that Americans in particular tend to exaggerate the meaning of bowing/curtsying, somehow equating it to one person acknowledging his/her inferiority to another. To me, it is just a socially acceptable form of greeting someone who holds a certain rank or title. In other words, in the same way it is appropriate for an active military person to salute another person in uniform, or for a Catholic to kiss the hand of a bishop, it is equally appropriate to bow/curtsy to a king/queen or a prince/princess. In any case, as you said, bowing/curtsying is always voluntary and it is not like you will be thrown in the Tower of London and have your head chopped off if you don't do it !