Bit of both in my experience, mine is a 2011 with ~33k miles on it when I bought it, exhaust tips looked pretty blackened/corroded but a blast with a rag and some metal polish soon put a proper shine on them like new, a lot of people probably never actually wash the tips when they wash the car so they just go black over time with the exhaust soot build up.
I wash my car probably every week or two and after a week (~300 miles) of driving the inside bottom of the tips tends to have a thin layer of soot on that wipes off with a cloth, wouldn't spend too much time worrying about the condition of the tips unless they are legtimately rusty/damaged underneath.
*edit* missed the "reason" bit, I can only imagine it's caused by the car running rich at various times under high boost or alike. The design of the tips also doesn't help matters compared to your standard straight thin exhaust tip as the ends are rolled over and form a nice lip on the inside for particulates to build up on.