There's an economic incentive for the moon base! Since it's so expensive to bring heavy payloads up by rocket we can just mine them on the moon. It surely won't be too long until the US needs to mass-produce the godrods anyway.
What matters is whether enough money flows in to buy elsewhere what you can’t produce.
If (a big if, but an assumption in Heinlein’s novel) a moon base can be made profitable, and kids get born there, so that people end up thinking of themselves as citizens of them moon, would it be fair if most of those profits keep ending up on earth?
In Heinlein’s novel, the idea is not that those living permanently on the moon bomb those on earth (even though that does happen), but that they use the threat of bombing them to get a fairer deal.