im not sure its fairt to even ask Lucas why Vader has to obey his master, he doesnt know either. the real 'mystery of the dark side' Adam, is found in the old trilogy not in the new ones where the mystery could be cleared up or founded in some way, but Lucas just fucked it up.
the real enraging thing about the prequels is not that its the worst thing ever, but that its so close to touching on what could have been awesome that it just fails all the more.

is Adam a gimmick? or is he Lucas in disguise? cause no one can be that stubborn.