wellll....
if you think about it logically, they want their citizens to be "free"
and of course, as always in war, propganda about the "barbaric, child eating, women raping, men slaughtering aliens" (aka you) is bound to happen.
they hate you. no reason to give planets to you. Give planets to someone else and hope they survive.
although yeah, i still cant think historically... maybe because if they surrender they're already dead?