Those races are meant for the people that want to "specialize" in good or in bad, and are not interested in playing a bit of both.
Reward is having a more rare race that not many will roll due to the difficulty of unlocking it, and being locked into one alignment, and for those that want to play only good or only bad, being locked in doesn't matter.
As reward they get to play a rare character not often seen around the world.
Also being "locked in" to alignment does not mean that your game play is limited, at all. As an angel you still get to attack any evil character and even any good character after he committed a crime of any kind.
And as an abomination you can still behave as a decent person, even if your alignment is evil. You will always be a free target for anyone, as evil, but if you are PvP player, then not being able to "switch to good, and escape part of potential attackers (from beast world side)" will not matter anyway.