While I agree with you to a degree most airlines and hotels have APIs that can be consumed to get pricing information there are just restrictions in what you can do with that information.

Not sure about stock prices (I think it's pretty common to pay for real time data there?).

But I can certainly see sites that have a lot of data for their users facing major bandwidth costs if a lot of people were scraping their data. This type of detection isn't really an answer for that, though, as it's easy to mitigate for a scrapper.