For the last six years there has been a Craigslist API, but it's not the one developers have been clamoring to get their HTTP requests on. It's still likely an important part of the company's business model, because it allows bulk posting to its real estate section, the site's main source of revenue. Yeah, but what about that readable API everyone wants?
Usually I'd consider a read only API to be the base feature. For example, the Google Plus API does not allow posting. It's been an intentional decision, apparently based on the value Google keeps by not opening its social network to just any feed of data.
On the flip side, Craigslist apparently sees more value in being the only source for its listings. So, it has had a writable API since 2006, skipping over the readable API. The company does have RSS feeds, but has strict requirements for their use and shuts off those who abuse the feeds.
The web's first map mashup was built on Craigslist and Google Maps in 2005. Some see HousingMaps as having forced Google's hand to create the now-popular Google Maps API. Clearly Craigslist didn't get the same message.
Much of the recent outcry has been over PadMapper, the spiritual successor to HousingMaps (which itself is still working). Craigslist shut it down in June, but then it used the 3taps API to bring the listings back. Then Craigslist shut them both off for "unabashedly mass-harvesting" postings and brought the matter to court.
Developers want a full-featured Craigslist API. Many are even willing to pay for it. But for now, it looks like the company thinks there's more value in keeping that data on their own servers.
APIs can help spread value beyond just the areas where a company has control. But an API needs to make sense with the business. One of the greatest challenges they've faced historically is the concern that an open API will cannibalize current revenue streams and jumpstart competitors. We've seen this concern at Twitter as it tries to figure out what its service looks like as a business. And clearly, seven years after HousingMaps, Craigslist has still not found a way to distribute its value via API.