Craigslist is a company whose terms of service were violated by PadMapper. Whether or not PadMapper provides a better experience is irrelevant: Craigslist has a right to defend its content and lists by whatever means it sees fit. Any businessperson on this site would expect to have the same rights.
Craigslist is not in the business of helping anyone but Craigslist be successful. It's not monopoly; it's business.
Companies are not free to do whatever they want. The government compels monopolists to avoid anticompetitive behavior, and even breaks companies up, for examples. The market in this case is a little hard to define, since apartment seekers aren't purchasing the product, so I'm sure it's not actually illegal, but it sure seems to violate the spirit of these laws, which are designed to protect consumers from exactly this kind of behavior.
Craigslist is not in the business of helping anyone but Craigslist be successful. It's not monopoly; it's business.