alt Hacker NewsCraigslist doesn't make any money from those scams because they don't charge for rental listings. It sucks that it's there, but for them to hire staff to deal with it, they'd have to charge for the rental listings.
Right now they rely on volunteers to combat that problem, in the form of legit landlords reporting the scams.
So why not charge for rental listings? i'm sure the number of scams would go down, while posting would still be of good value for someone looking to rent out a $3000/mo apartment.