I don't think this is what's happening here - for either Krrb or Padmapper.
It's important to note that Krrb and Padmapper both make the distinction that they're scraping facts, not posts. One is copyrightable, the other is not.
You can lay claim to the sentence "I have a room in a 2-bedroom apartment for rent in Williamsburg, going for $1500"
But you do not have the right to own the fact that there's a room for rent in Williamsburg, in a 2-bedroom, for $1500. It would seem that Krrb and Padmapper both scrape the facts only, not textual content - or at least, that's what the letter seems to say.
This is a frequent misunderstanding of the Padmapper issue. Padmapper never scraped posts wholesale from Craigslist - it scraped and displayed only structured data (i.e. facts), and in fact if you wanted to contact the seller it links you directly to the Craigslist post!
Does it matter that they are using Craigslist infrastructure to automate the process? Consuming bandwidth that Craigslist pays for and taking advantage of the hosting Craigslist is offering their paying customers? Sure, facts are not copyrightable, but if you took a stack of phonebooks from a recycling bin and starting selling them as your own, I think the company that printed the phones books might have an issue with that.
As I understand it, Krrb is not using Craigslist infrastructure - it is simply taking the post already in the user's browser because they navigated there in the process of entering the post, and copying the text out for reuse.
Craigslist can only litigate on that basis because they know Krrb doesn't have the means to defend themselves. That's not a public good.
The post I was replying to was grouping krrb and padmapper. I don't know much about krrb or what it does. Seems like a glorified copy past so I'm not sure how that exactly translates to a business worth putting this much effort into defending. Maybe I'm missing something.
But padmapper is clearly leveraging Craiglist's infrastructure for padmapper’s own benefit. I find that difficult to defend.
Plus there needs to be a certain level of creative expression for copyright to apply, so even the sentence, "I have a room in a 2-bedroom apartment for rent in Williamsburg, going for $1500," would almost certainly not be copyrightable. (That said, some of the posts on craigslist are certainly creative, so copying the full contents of everything would likely violate copyrights, but as you say, that's not what Padmapper is doing. (Not to mention the copyrights on the individual posts belong to the users, not to craigslist, in any case.))
It's important to note that Krrb and Padmapper both make the distinction that they're scraping facts, not posts. One is copyrightable, the other is not.
You can lay claim to the sentence "I have a room in a 2-bedroom apartment for rent in Williamsburg, going for $1500"
But you do not have the right to own the fact that there's a room for rent in Williamsburg, in a 2-bedroom, for $1500. It would seem that Krrb and Padmapper both scrape the facts only, not textual content - or at least, that's what the letter seems to say.
This is a frequent misunderstanding of the Padmapper issue. Padmapper never scraped posts wholesale from Craigslist - it scraped and displayed only structured data (i.e. facts), and in fact if you wanted to contact the seller it links you directly to the Craigslist post!