If it's mail, they probably know not a lot of programs around still have the proper setups or are willing to accept it. This is probably why they believe they have leverage over you.
Also, if they have other large mailers, 20-30 joins per day truly isn't a lot for mail. Large would be at least 70/day (even in today's climate).
That said, if you can send somewhere else that accepts the traffic and converts along the same lines, you should. You don't have to put up with attitudes from reps/programs.
