You're mostly correct. It's a racket for the state in MA, but in NH, the mechanic only pays a few dollars per sticker to the state.

Even at $50, the mechanic isn't making a lot of profit, compared to their normal labor rate. As you said, some dishonest mechanics use the inspection to sell unnecessary repairs. On the other hand, honest mechanics do make money off inspections when they recommend repairs that are truly needed, and only when they are needed.

On the one hand, I'm amazed and saddened at how many people take such poor care of their vehicles, and it's disturbing to see people driving with bald tires, burned-out lights, etc.

On the other hand, the actual data show that most crashes are caused by driver error, and very, very few crashes are caused by vehicle defects. You're definitely correct in your stance that vehicle inspections help very little, and should probably be abolished.