US will have different laws and regulations regarding specs no doubt. Their petrol is different for example.
They likley changed it because 6 months is unlikely required for their cars. I'd do 6 months if I was driving an expensive performance sports car like a Porsche 911 turbo or an Audi R8.
There's a reason why you can have two of the exact same car and one falls apart after 100k miles why the other keeps going for 350k miles.
Regular servicing and 2 years is far too long. Car engines are expensive and lots of moving parts. Spending £50 a year on top quality oil and half a day changing the oil and filter yearly is hardly a huge outlay. Even better if you have a local independent who if you provide him all the fluids and filter will do it for £40-£60 cash.
You could also ask why if the legal requirement is to change a tyre once it's 1.6mm a lot prefer to do it at 2mm.
It's performance has degraded significantly at 1.6mm same with oil after 2 years or 18k miles.