Because when they give their SSN, they probably aren't planning to quit, unless they're really bonkers.
It's sad that you can't trust someone's word, but a lot of us have been shown (in real life and on here) that you can't. Also, sometimes I think that it is innocent - the women who joined a second group weren't properly educated during their pledgeship on sorority and NPC rules and regs because they were too busy opening their million gifts. But that is another thread
.