I noticed this, too, but I wasn't sure if it was just natural aging.
One thing to note: you will never see dressing on race tires. IIRC, it has a tendency to harden the rubber, which I suppose, could lead to cracking. On a race tire, this could be a performance issue if the stuff is getting on the actual tread. For a street tire, it wouldn't matter as any "damage" would not cause tire failure, and the customer wants shiny tires. Doubt there's any lawsuit danger.
Anyway, just speculation.
