Unfortnately this does happen in the industry, you just gotta be a smart customer and makes sure its clear what you want done and that you must be informed about anything extra that may have to be done so you can approve it. Problem is majority of people have no idea about cars and usually have no argument or anything to say to extra or unnessercery things performed on thier cars.
This kind of unethical behaviour is not just in the automotive industry, business today is rort with unethical behaviour from all area's espeacially from a economic and enviromental aspect. There is a 3 level measure for corparate ethics undertaken a few years ago and 90% of them measured at a level three which was the worst.
Its just unfortante that the Automative industry gets a rough rub because of alot of pre-mature presumptions from alot of bad word to mouth incidents mostly about mechanics.
I cant say i havent had bad experinces, but majority of the time I have got good service and helpful advice. Tommy you just have to be extra vigilent in the furture from your bad experiences. Make sure you ask around find who has the best word of mouth, look round for best prices, makes sure you ask questions and make it clear what you want done and most of all if you are not satisfied "Complain" (in a nice manner of course) let them know your problem and how it is going to be resolved. To many people cop it on the chin when they shouldnt have to. Last resort if everything fails complain to the Fair Pracitces Commission.
I dont condone the Un-ethical behaviour just take precautions. I have met many great mates through the industry, alot of them were Datsun related people in the industry because it makes it eaiser if there intrested in your car and have the same intrests as you do towards for make of car