Tom Evslin posts with several reasons why Web sites don't provide APIs -- and yet predicts that many more will provide APIs in the future. I can add one more reason, and that's the risk that with an API, somebody would build an application that presented the data through an alternative UI, and the site would lose eyeballs. For example, if Yahoo Group calendars could be sucked down in iCalendar format, people would probably visit the site less often and receive fewer ad impressions.

Still, I agree with Tom that despite all these forces against opening up APIs,
there are even stronger forces for having APIs -- competitive advantage. Some company hoping to compete at lower cost will provide the API and try to make up the revenue in other ways or simply survive with less ad revenue. If the service is more valuable with the API people will move to that service. I hope that in a year or few, people won't stand for a calendar Web site that doesn't let them use a standard API to have direct access to their own calendar data.