No one is suggesting they open up USSD for free, Econet can charge well above and beyond what the extra infrastructure would cost them, where it is necessary, already econet are charging a fee per session to check your balance etc on Textacash but USSD also uses insignificant amounts compared to other data services.
Basically Econet want to block the banks from transacting while demanding the banks let them transact. Play ball and open up your platform if you want them to open up theirs.