I would guess (or hope) that a small part of the decision is that the company does not want the liability of something happening to an F/A once they leave the aircraft, carrying around large amounts of money until they get to a base they can deposit it at. I know if you land at one of our outstations, you are forced to bring any cash that goes from onboard purchases with you to your hotel and that made a lot of people nervous!
I know personally, I have spent many Hawaii layovers with $500+ in my bag and wasn't comfortable being held accountable for money that I had to hold onto for almost 3 days. Many crew members have had personal items stolen out of their hotel rooms, most likely by housekeeping or maintenance. It got to the point where I would lock my rollerboard with anything valuable I had, just to err on the side of caution, but there's nothing stopping someone from taking your entire bag.
There's also instances of flight crews being mugged on layovers and having to file police reports. In those cases, how could the company hold them accountable if it was a circumstance beyond their control?
As previously stated by others, it seems a large portion of the revenue increase will come from those employees that won't be able to steal cash any longer. It's a sad fact but certainly not just limited to US Airways, there's those employees at every airline. Unfortunately, that's quite a bit of lost revenue that in the end, hurts us all. Hopefully this is a change that will benefit customers and employees in the long run...