I tried to build a flight search site a few years back and many sites have measures in place to actively mitigate scraping. It quickly became obvious that I was going to spend more effort getting data than working on the product, so I scrapped the idea.

Like you said, scraping is against ToS so as soon as you get caught, you're cut off until you shell out $$$$ to buy a feed. I suspect the reason they've succeeded here is that they've stuck with a human-based approach.