There are many great examples of Google failing to internationalise, but Google Fi requiring a US billing address is not one of them.
Fi is a domestic cell phone service, it provides international roaming as a bonus to its US customers - at much increased cost (to them) compared to domestic service. I wouldn't be surprised if roaming permanently was a violation of their terms of service (it is for Three in the UK, who provide a similar perk)
You can of course criticise Google for not launching domestic service in countries across the world, but in fairness to them, that's not a small problem.