Do you mean that an authorised SP shop will refuse servicing or honouring the guarantee for their own products?
This does not happens here for products purchased directly in the USA (Apple, for example, or Dell).
When a product is sold with a "worldwide guarantee", it should be serviced all around the world, this is actually one of the best selling point for SP: their regs are supported by a worldwide network of shops, all carrying spare parts and which can fix you reg while you are travelling in exotic locations.
Other excellent brands cannot say the same, outside certain countries it is almost impossible to find shops capable of servicing them.
Regarding import duties, these are generally a very bad thing, and I warmly hope that all these additional taxations will disappear in this globalized world. Removing them within the EU market boosted the economies of EU countries, so I really hope that this unjust taxation for importing goods is removed worldwide.
In the meanwhile, however, there are ways to minimize these costs. I do not know the tricks for the route EU=>US, but I know a number of tricks (some fully legal, some not so much) for avoiding to pay high import duties when I purchase goods from US to Italy. A quick search on the Internet can show you some of these tricks. As said, some are fully legal!
You can get SP equipment serviced anywhere in the world but warranty is something else. If you purchase something in the US from an authorized dealer, you get warranty from SP but buying it from an unauthorized dealer or from a SP dealer not authorized to sell to the US, you won't get warranty. Covered by warranty and getting it serviced are two totally different things. It is just like if I buy a Ford truck in the US and bring it to Libya, the local Ford dealer won't cover any warranty repairs by warranty since the warranty is restricted to servicing in the US. The dealer in Libya will be happy to service it but they won't cover anything under warranty if you bought your car in a different geographical area. If I want valid warranty for a Ford truck, I have to buy it from an authorized dealer for the geographical area where I'll be using the truck. Same thing applies to most car brands (Mercedes may have different policies). Computers, Dell or other brands, are the same thing.
Manufacturers have to protect their dealers in various geographical areas and enforce some sort of restrictions including restrictions on warranties.