I actually caught a similar scam on my old Mitsubishi Mirage. I accidentally scored the oil filter by dropping a wrench on it one day. I took the car in for service, and they kept it for about 4 hours. When I came back, the same oil filter was in place, but more surprisingly, the air filter was GONE.
When I complained, they pretty much called me a liar, saying that I took the air filter out and damaged the oil filter doing it.
So no, dealers aren't immune, they're just as susceptible to scandals as any other place, they just charge a lot more to screw ya. Find a good, local, ASE certified mechanic and befriend them. They'll do ya right. Stay away from the dealers.