Comment by EMIRELADERO

Just because information is public doesn't mean it can be used without restriction, similar to how certain open source licenses doesn't allow you to do anything you want with the code just because you can view it on GitHub. For example, website owners will realize this quickly when they copy an image from Google Images and get a lawyer letter demanding payment for unlicensed usage.

In this case, Yahoo licenses the data from various exchanges. I am not sure which exchange they use (some sites often use a cheaper exchange like one of the CBOE exchanges, rather than licensing the entire consolidated tape, which is the combination of all exchanges and gives the most official and up to date pricing). The exchanges are very particular about distribution rights and sites pay a pretty penny (we are talking probably tens of thousands per month for a single data source) to access it. I am not sure exactly what legal method the use to "shut down" unauthorized, access, but they can and do depending on how serious they are (for example, NYSE may be the strictest and does regular audits).

Of course, exchanges understand that their intellectual property can and will be reproduced when it is offered for "free" through public websites, so that is factored into the cost that websites pay and for the most part everyone turns a blind eye to small personal use. However when you use it commercially or build a project (even free) around their IP, you can expect them to take a second look at some point. Exchanges make a killing on this I am sure, as they charge admin fees, monthly data fees, fees per user/visitor, and all kind of other clever things. To learn more look up "<market> Fee schedule" (where market = CTA or UTP for consolidated US equities, OPRA for options, CME for futures, etc) and you can find the licensing rates that most exchanges publish (which doesn't include the rates you will pay to a data vendor to actually get the data).