It's becoming pretty standard behavior to employ lazy loading, as you noted. Speed and responsiveness are important to the user experience. Any design decision is based on the tradeoffs between the different choices.
In these examples, the user's behavior dictates when additional content is loaded, but the initial load is very fast. The screen says "here's my content" and when the user responds by scrolling, they are saying "Great, show me more." The system responds by loading additional content. It's like adding social skills to the web app. It's waiting for cues from the user that more information is welcome.
If this design decision means more people will see featured content, that could be worth $$ to advertisers. This strategy can keep people on your site when they might otherwise not wait for the page to load and might abandon the site.
You mentioned you wanted a button press to load the additional content. I don't see any advantages to doing it this way, when scrolling is expected behavior and the user will likely scroll anyway. The only reason I can see for using a button to initiate this behavior is if you have to obtain the user's consent to show the information.