Personally, my intuitions change pretty heavily in what I see as a monopoly situation -- especially a natural monopoly based on network effects. So here's why I support Padmapper, in elaborate detail:

Part 1: Craigslist has a natural monopoly. Let's assume (as I believe) that in my city 90% of by-owner apartment listings happen to be posted only to Craigslist at the moment. Since apartment hunters have limited time, most of them (like me) search only on Craigslist. Since landlords have limited time and most apartment hunters only search on Craigslist, most landlords only consider posting on Craigslist. Which means the cycle continues, and next month 90% of by-owner apartment listings will still be posted only on Craigslist.

Part 2: the natural monopoly makes Craigslist prohibitively difficult to disrupt. The sole valuable commodity in the marketplace is information about which apartments are available. Craigslist obtains this commodity for free. Craigslist gives away this commodity to consumers for free. Craigslist refuses to share this commodity with anyone else. So disruption is highly unlikely: you would have to generate a commodity (information about apartment listings) at great expense that your competitor gets for free, and then you'd have to make money on it when your competitor gives it away for free.

Part 3: Craigslist is taking advantage of its protected position by offering an inferior product. It is the only real estate site on the internet that fails to take advantage of a certain infographic tool developed several millennia ago to represent the most important fact about real estate in a way that humans can easily understand. Which is to say, it doesn't have freakin' maps. This has wasted, literally, many hours of my personal time (longer than it took to write this comment, for example), and the time of hundreds of thousands of other people like me, and it is not a choice that a company facing any kind of competitive threat would make.

Part 4: Craigslist's protected position depends on our recognition of its ownership of public information. Landlords are using Craigslist to announce to the world that a certain property exists at a certain location for a certain price. The basic state of a real estate market is that there are a bunch of people trying to advertise their goods, and they'll use the most efficient way to do it. If we as a society decide that, because Craigslist is the most efficient way, it therefore has exclusive ownership of the information it is broadcasting, then Craigslist can continue to use its protected position to offer an inferior product without competition and waste hours of my time. If we decide that Craigslist does not own the basic facts it is broadcasting, then the information becomes a public resource, and services will have to compete to make it conveniently available, causing the market to work better.

Given the above four points, I conclude that the information landlords broadcast through Craigslist should not be Craigslist's property, because propertizing that information causes harm to our society that isn't outweighed by the benefits. So I support Padmapper's choice to test Craigslist's legal right to control that information, and I hope it turns out that Craigslist can't.

The world always turns out to be more complicated than I want, of course, and maybe there are terrible things that would happen in other situations or industries -- or even in the online real estate market -- if Craigslist didn't have the legal rights it claims it has. I'm open to hearing that argument. But if you're trying to understand why so many are defending Padmapper, I hope this helps.