Pretty sure it's from high heat. The pitting is only in the middle of the grate where the baskets were placed. The underside also has caked on black carbon that's about the same radius as the pitting. But I agree, it's still perfectly usable and will likely last awhile once it's seasoned. I really need to find time to start grilling!
BTW, I just realized Weber used to use stainless steel ash sweeps on the upper models of Weber Kettles, while the lower models had aluminized steel sweeps. I haven't checked mine with a magnet yet, but it's probably 304 stainless like the other stainless parts on this grill. According to the Weber catalogs, they switched to aluminized steel partway through production of the Gen 3 Performer, and now aluminized steel is the only option.
Oh, and I was at the local hardware store today. They had two Kamado E6's on clearance for around 40% off. It was a steal of a deal, but I just couldn't justify the purchase...