Patents are 'territorial'. They are only valid in the country the patent was filed and granted. Companies can protect their patent(s) in other international countries but it is expensive.The issues is not Apple or Masimo, it is the patent office. Obviously Masimo was not able to get the patent offices outside of the United States to issue patents, because Apple is able to sell outside of the US without patent infringement. Having been responsible for patent acquisition and enforcement, it became clear to me that US patent office is a royal mess.
This website gives you an idea of patent protection.

Worldwide Patents Are a Myth, But Here’s Your Real Strategy
Think you can patent your idea worldwide? Think again! While worldwide patents don’t exist, there are effective ways to protect your invention from international competitors. Learn about PCT applications, targeted market strategies, and how to navigate the complex world of global intellectual...

This is a snippet from that website:
Currently, we don’t have an international system that protects and enforces patents globally. Patents only protect an invention in the country where they are registered. For example, if you patent your invention in the United States, then your invention is only protected in the United States and it cannot be enforced outside the US.
To obtain “worldwide” patent protection you’d have to file a patent separately in all countries in the world - which is pretty expensive, time-consuming, and, in most cases, not possible.
Basically a company would want to protect it's biggest market. The US is Masimo's biggest market thus it stands to reason to file it's patents in the US. As you can see by the quoted section from the website, Masimo could file patent protection in other countries but it would be expensive for them to do so. No point if sales are small in other international countries.