AFAIK, most of the cost of opening the back account goes towards the Bangkok Bank insurance policy.
I paid ฿3,700 two years ago because I'm in the lower age bracket, and that covered the insurance policy plus an opening deposit of ฿521.
So you're actually getting "something" for your money and not just lining the agent's pocket. If they get anything it's not much.
The OP could, of course, try to open a Bangkok Bank account on his own but that involves a lot of walking around and waiting in bank branches to likely be told "cannot".
Yes, I know you know this but just reiterating it for the OP's benefit 😁