Every website is routed through Facebook servers. They created a pseudo-standard as opposed to standard HTML and only the websites with that would be allowed onto the program. Imagine if Google comes over and says "AMP" loads faster, takes less data is better but, is a subset of HTML spec & "Google"-sponsored format; So, we will give free internet access but, only websites which adapt to our specifications would be allowed and not everyone. You either provide internet access or no. Facebook could have gone with tiered system in regard to Free basics as in provide free data only for about "X" GB. Most of India consumes internet in terms of amount of data. They didn't go about this approach but, rather wanted to get on with providing free internet but, collect data of the website visit information, hava user analytics.

Another important consideration is that Facebook only allowed any website to be in it after initial backlash. Initially, it only allowed websites deemed necessary by Facebook like "facebook", "messenger", "groups" and "Google. No way for users or website owners to say they'd like a wesbite "xyz" as part of program. This was and is an attempt by Facebook to create a walled gatway to internet by Facebook masked by their philanthrophic arm.