Earlier quoted context omitted.
Why would they do that? That's what the APIs are for.
Facebook already updates their API and policies very regularly, nothing they give an application access to can be given for granted in the long term. Especially when it deals with private content. When your service is dependent on another, you're always at the mercy of an update that can shut down the API feature you were using, whether the intention is to kill your service or not. In this case it seems like the goal…
> In this case it seems like the goal of the website is to become a starting point for a person's browsing/searching experience, facebook and google want the same thing.
This is your only 'why', but it's very weak. Every company wants to be the 'starting point for a persons browsing experience', but Facebook doesn't provide that, and neither does Greplin. Google does of course, but Greplin doesn't.
Your point reduces to "companies which consume APIs might compete with the companies which create them, and so might be shut down". Of course, your statement is true, but it's valueless.