They hate it because it can get you a very good price. The deal's not done until you drive off in your new car. You don't have to be a jerk about it, but you should be on guard throughout the entire car buying process. The biggest place that people err is when they have struck a deal with the salesperson. They breathe a sigh of relief and think the dealing is done.
Extra charges can appear in the contract.
There may be a problem with the condition of the car. All these things can be addressed if you have the right attitude. Always remember that it's your money. Car salespeople are good at making us feel obligated to buy from them. They serve us sodas in the sales room.