Scott Base is easy walking distance from McMurdo, not 99 miles (or 100 kilometers and 100 miles and 100 nautical miles - that site is quite confused!).
Both are actually in New Zealand depending on the purpose, due to the Ross Dependency[1]. For instance, most of those folks in McMurdo station have longer NZ visas than typical US tourists visiting NZ would get (3 months), because when they fly down to The Ice they're not technically leaving the country.
An interesting corner case is when something happens at one of the US stations in the Ross Dependency, which New Zealand wants to investigate. The Rodney Marks death[2] is a particularly well-known example.
[1] https://en.wikipedia.org/wiki/Ross_Dependency
[2] https://en.wikipedia.org/wiki/Rodney_Marks