Just a guess — but I imagine that Starlink passing over a continent and not having any customers below would be a waste of that orbit arc. I mean Starlink could just give away the bandwidth until it actually was running low on it.
The only caveat I'll say is that starlink generally requires ground stations to provide connection at scale. So it's not 0 marginal cost for them to provide it for free. But the general thought is right: the marginal cost is small. Launching satellites is the expensive part, and once you have them up there, you might as well serve Africa