Technically speaking you can scrape data in a legally defensible way if you do not need to accept any terms of service explicitly prohibiting scraping in the course of grabbing the data. The distinction is that browsewrap T&C have plausible deniability, but clickwrap T&C do not. And if you receive a cease and desist order, you abide by it with a mea culpa. This also means you don't scrape so loudly as to be noticed, which has the happy side effect of probably not disrupting the target's service.
But again: The grey areas of ethics are a separate question from legality. Please engage a lawyer for your specific work.