Exactly. Craigslist isn't randomly opening their local sites. They are doing so in response to changes in communities following some semi-strategic growth strategy. Its not a natural experiment as claimed by the authors at all. Its like looking at the effects of google fiber on the cities it launches in without taking into account that those cities were selected by google for a reason. Quasi-experimental methods would be helpful here like a propensity score matching to match the cities to others that are like them and make pseudo-controls.Even then you can't consider this remotely a natural experiment.