"Market research by suing those who try to improve you? Doesn't seem like the most effective system."
Let's be logical here.. Craigslist may have been planning this feature for a year when Padmapper began its service. Craigslist would be stupid to allow them to continue to piggy back on them and become popular when they have already planned rolling out a service like that.
Even if they hadn't been throwing that idea around (unlikely), you still absolutely do not, as a business, want a 3rd party to be able to 'represent' your product. In this case, Craigslist would have no control over how Padmapper presented user's with the Craigslist content. Controlling you brand is SUPER important.