This might be a naive comment, but isn't that predatory pricing. By dropping prices to zero and trying to purposely hurt your competitors, you effectively gain a monopoly on that industry.
Github's competitors are already providing the same service for free. Github is dropping their prices from $7 / mo to free to compete.
But they're not. They created a new, feature-reduced repo type to make free/private. The marketing hides this, and you need to dig to the "Comparison" section to see it:
Good point, it's not the exact same. But their new offering is much closer to what their competitors are offering, so I'd say it's still a move to try to better compete with the free offerings from other companies.