The reincarnation thing certainly seems to have caught people's attention -- rather out of proportion to the attention paid the concept of social justice. Seems American newspaper readers are very concerned with liberty and equality, as long as they get to be more equal and free than others.*
It's not a metaphysical proposition; it's a thought-experiment:
We don’t have to believe in reincarnation to fight for a world that we’d actually want to be born into.
Peoples who live in relative material comfort and security tend to relax their clutch on religious dogma, have fewer children and do a lot less fear-biting than peoples who live hungry and under threat of violence. The comfortable and secure also function better intellectually (stress consumes a lot of brain-activity) have more education, more leisure time to become informed, make sound political choices and solve problems. The more people live in security and comfort, the less strife and crowding, epidemic disease, territorial conflict, resource shortages and oppressive political regimes.
Pretty simple, really.