From the MTH Website...
Authorized Service Centers (ASC)
Authorized Service Centers will provide warranty work for any consumer who has purchased an M.T.H. item from them and after-warranty work for any customer.
National Authorized Service Center (NASC)
National Authorized Service Centers receive an extra incentive from M.T.H. and will provide warranty and after-warranty service for any customer.
In short the ASC only do warranty work on items purchased there. A NASC should do warranty work regardless of where it was purchased but I am guessing it had to be bought from an authorized dealer.