Is service engine light a scam?

The service engine light on my 2006 Nissain recently came on. We took it to our trusted mechanic and he was puzzled as to why it came on (the code was for a transmission problem that is very rare, especially on a car with less than 30,000 miles). He reset it, and told us that if it comes back, to take it to the dealership as it may be covered by the warranty. It came back so we went to the nissain dealership. It was $120 to check out the problem. They told us that nothing was wrong, and "reprogrammed" the service engine light. I repeatedly asked WHY the light came on. The dealership thought I should be happy that "nothing was wrong with the car". To me, having a light randomnly pop off that requires a $120 fix is a complete and utter scam. If it's "scheduled maintanence, then they should explain that...instead of simply charging me money and telling me I should be happy. Are these lights scams?