The problem for Craigslist is that the one thing people want is a better interface. If CL provides an API people can use to build an alternative frontend, and users start to go to these frontends instead of CL itself, there's nothing to stop those new frontends from extending CL with their own listings, and eventually extinguishing it. That was PadLister's business model.
Padlister/Padmapper doesn't make money from from CL listings. Padlister charges money if listers listing directly on Padlister decides to use the online application.
