Some people would say that consumers are free to avoid vendors that don't accept cash, but in practice this "freedom" is actually a huge cost, in terms of time and mental overhead and lack of choice.

The danger is that society as a whole ends up making sub-optimal decisions because each person is (selfishly / rationally) choosing to avoid these extra costs.

Powerful entities (even those falling short of absolute monopolies) probably get away with a lot more societal harm than they would if we were more aware of this "coercion of convenience".