Just kind of thinking out loud, but most businesses rely on word-of-mouth to get new customers and good service to get repeat customers. Dealers should treat you good both on the sales floor and in the service bay. I know that not all of them adhere to this, car salespeople are among the least trusted groups and I think that's because they are aware that if you leave the dealership without buying, you probably won't return so there's a lot of pressure to get you to spend a lot of money quickly. I let them know when I'm shopping, I don't make hasty decisions and I'll be back if I'm treated right. Any deal they offer today, they'll offer tomorrow or next week.
As for their service departments, they get paid by the manufacturer for warranty service, so that shouldn't matter what kind of problem you have, that's how they stay in business. Do some try to convince you that your problems are "normal", or non-existent? Probably, but they're turning away revenue they need when they do that. Do some "fix" problems that don't exist? Yeah, there are some that try to increase revenue one way or the other, but then, they get a bad reputation quickly, again, there's that word-of-mouth thing that can get them outed, especially in this day of the Internet where opinions expressed on social sites and forums like this one can help or hinder business.
Just my thoughts, I've bought a lot of vehicles over the years, and there are some dealers I wouldn't go back to both for sales or service just because of they way I was treated.