If a court decides the usage wasn't legal, what's your liability as the service that provided the crawler?
We consider ourselves only as service providers. We provide technical means to perform the crawling but the responsibility for the actual crawling is with our users. It's similar to Amazon AWS - they only provide infrastructure and it's your responsibility not to use it for anything illeagal.
Just curious but how to do enforce this assigning of responsibility? You make the user agree to terms of service before crawling?
Yes, every user has to agree with the Terms of use when signing up.
How does that gel with features specifically aimed at cirumventing scrape-detection and blocking such as IP rotating?