Sigh, antitrust is a dead end. It might have been possible to argue prior restraint if Craigslist had stuck with their silly 'we own this' clause but they didn't.
My guess is that any judge would say "Can anyone else make a classifieds site?" and "Does Craigslist interfere with anyone on their site using a different site?". Since the answer will be 'No' and 'No' thus they don't have a monopoly.
That seems overly simplistic. Anybody could have used a different browser/OS (there were several available in the 90s), yet MS was still found to be violating antitrust laws.
The real question is whether Craigslist is using its market dominance in a way that hurts consumers. It's a far more nuanced question and one that will be fascinating to watch get answered. Craigslist did some awesome things for consumers once upon a time. Now? I'm not so sure it's all positive.
Your counter-example (ms windows+expolorer) is not-quite on point, though. Consider: windows had a legitimate monopoly, and then they "abused" it. But, because you are presuming the existence of a monopoly for CL this does not work. CL is an online classified ads business. The barriers to entry are de-minimus (unlike, OS software + hw compatibility, ect). So the premise of monopoly can be attacked fairly and directly.
CL arguably is not abusing a monoploy because they don't even have one. They just have a cheap service, and nobody wants to pay to do it different. The consideration you raise -- are they anti consumer -- is an interesting one to think through. It does seem they over-reached (re: the exclusivity of data). But without any contractual exclusivity or confidentiality its not clear that CL can "monopolize" publicly published data, given that the most valuable bits are (at least for rentals) nothing but street adresses, apartment #s, and possibly Telco #s. These data are not legal-sense "intellectual property" under most concepts (patent, copyright, or trade secret).
It would be an interesting argument to see the threshold case: when/where do 3 facts become a creative work? Its not a trivial argument, though, because at some stage N words becomes a lyric a poem or an idea. You cant copyright a word, though. Its highly unlikely you can copyright an adress. They might try to argue its like a "recipe" or some such (list of common ingredients?). Who knows. But somewhere in there, I'm guessing we'll find out.
Well if you recall Microsoft took an additional step, not only did they bundle their browser with windows, they put language in their contracts with OEMs that forbade them from putting other browsers into the system. Had Craigslist stuck with their idea that you couldn't put your listing anywhere else, then yes you could have argued they were using their position to restrict trade, but they stopped doing that fairly quickly (according to the article after talking with the EFF).
My guess is that any judge would say "Can anyone else make a classifieds site?" and "Does Craigslist interfere with anyone on their site using a different site?". Since the answer will be 'No' and 'No' thus they don't have a monopoly.