Ask HN: I want to build software on top of existing websites, where is the line?

It's a tricky area as evidenced by cease & desist letters from craig's list among other things. For example, if I want to make something that helps people do some additional processing on the information already available to them is that okay?

Take CL's TOU:

"USE. You agree not to use or provide software (except for general purpose web browsers and email clients, or software expressly licensed by us) or services that interact or interoperate with CL, e.g. for downloading, uploading, posting, flagging, emailing, search, or mobile use. Robots, spiders, scripts, scrapers, crawlers, etc. are prohibited, as are misleading, unsolicited, unlawful, and/or spam postings/email. You agree not to collect users' personal and/or contact information ("PI")."

Say I make an app that helps people organize the craigslist postings they are considering responding to. It sounds like my users would be users by violating the TOU.