You can cap the effective margin with even a very generous limit — say, processing fees no more than 3x the cost of servicing chargebacks — and mostly eliminate the concern about "quietly banning" businesses via processing fees.

But the thing is that there isn't much incentive to do this kind of skulduggery, because the companies are only protecting themselves from lawsuits and protests when they ban businesses, but imposing high processing fees would probably not satisfy the would-be tyrants who were trying to force these "bans".