Hello,
certainly the difference in the guarantee depends on the country in which it is stipulated,
here in Italy (and in the European Community) the mandatory warranty by law is 2 years, and the owner of the car can have any mechanic do the maintenance, even if not authorized, as long as he guarantees to work according to the specifications of the parent company.
Any longer guarantees, for example 6 years or 8 years, are always to be understood as 2 years of legal guarantee + 4 or 6 years of private guarantee.
So for the private guarantee (in this case made by Honda) it is a contract between private individuals and therefore you are forced to go to the authorized workshops under penalty of forfeiture of the guarantee (all car manufacturers do this), while for the 2-year guarantee provided by law you can go wherever you want to do maintenance.
I imagine that in other countries outside the European Community there are different laws.
(Sorry for the mistakes I used Google Translate)