What could be incentives for publishers to waive fees for reusing figures?
I've never had a problem getting permission to use a figure with proper citation – ideally such a usage drives traffic back to the original article / source too.  So that's one incentive right there.  Be sure to let them know you will send traffic / credit back and ask how they want to be cited and / or linked.

But authors ordinarily do not really expect to lose control over their IP, so authors would probably avoid such publishers if they found out that a journal "jailed" their figures. This is obviously a stick not a carrot, maybe saying something like "I think the authors would be very disappointed to find out if they have been disadvantaged by publishing with you, it might discourage them working with you in the future."