In a company, many conversations start this way: "Hey, we need more paid accounts. How can we further incentivize people to sign up?"

Maybe having a paid account has more utility to you with this added feature? Maybe you should sign up and try it out hint hint nudge nudge?

Another question is: "Hey, we want to provide a good program, but how do we make sure only to people with serious interest sign up?"

Restricting to paying customers is a good way of weeding out people that just sign up because there is a free sign-up form.

Presumably, if you're going to need the level of access provided by this program, you're probably building something that makes some amount of money. That would make purchasing a paid plan worthwhile, even if all your repos are public.

Why? Travis has tons of closed stuff running their paid product. So they are probably paying customers anyways.