It's the oils in the rubber of the tire coming out. Spray some degreaser on the tires (try not to get any on your rims) let it sit about 3-5 minutes them rinse it off. They will be back to black.
That sucks! These are the only ones on the market so far that I like the look of. Does the top edge at least have a lip that you could add some two sided tape to so it doesn't rub?
I have then same issue with the Security Alert, it worked fine from April till last week, but this week it tells me everytime the door opens. I turned it off this morning. My guess is that its a new bug in their system.
I mainly use it for remote start which works fine.