It's obvious they have no other mean for money.
It's not obvious to me, but then clearly that is just a difference of viewpoints or something. Angel f.ex. charged for his services, and I'm sure I could think of plenty of legal ways of making money if I had a whole bunch of superpowered girls handy. Like... construction! Xander could set up a firm (Since he knows the business), and they could make a small fortune building houses etc in record time. I just can't see bankrobbery being the only option. That's all.