My alloys were refurbed by the supplying non-franchised dealer when I bought it @28k / 3 years old as it was felt they were not upto standard ( bought from a well established Jaguar dealer).
I also have a spare set of 16's from a 2009 TS for Winter use. Both suffer rim corrosion.
Recently, after a long weekend driving on the 18's, I parked the car up and returned to it the followinfg morning to go to work and the TPMS indicated a pressure decrease. I inspected all the tyres and found the front OS had lost 10 psi. I pumped it up and drove to the nearest tyre fitter who found corrosion to be the cause of the deflation and resealled it.
I don't think Mazda, being built in Japan, unlike say Nissan, take into account the use of salt in Europe.
My 18's weren't on in winter so didn't see much exposure to bad roads.
This concerns me more that a corroded alloy could potentially deflate without warning than cosmetic appearance and at least the TPMS did it's job.