Dealers make good money on BG flushes and services, whether it is a trans fluid, induction, top end cleaning, brake fluid flush or whatever other service they perform. There's a lot of profit in it for the dealer and the tech not only gets paid his hourly or flat rate for the work, BG also pays the techs a spiff on all the services they do each month. Unless BG has changed its policies in the last few years, the way it works is BG puts a plastic chip inside each can or container of BG fluids and cleaners the techs use at the dealer. The chips used to look like a poker chip and they were different colors. Each color corresponds to the amount of money the chip is worth. When the tech does the job, the parts guy gives him the supplies for it, tech does the work, before throwing the cans and containers away, he opens the can lid and removes the chip. The tech saves the chips and about once a month the BG reps visit the dealer and the tech cashes in his chips for a cash spiff that the BG reps pay him right there on the spot. The service advisor makes money on the BG services he sells too. There is a lot of profit in BG services.
If you were unaware of the cost and type of transmission service they were going to do, and they did the BG flush without your prior authorization, it would be a good idea to take it up with the Service Manager. If he won't help you, maybe ask for the dealership owner or general manager. If they blow you off, then take them to small claims court. They have no right to rip you off and do extra and more expensive work just so they make more money or just because the service advisor recommends it. Your state may also have consumer protection laws that could help you on this. Personally, I would not give up on it until the dealer makes it right, one way or another.