I think that payment methods on the markets should be negotiated by the buyer and seller directly.
You have every freedom required start an exchange where that is the case, but it won't be the exchange that offers the lowest spreads. A successful exchange will keep the overhead low for the customer.
Payment providers vary a lot between countries, continents, and even languages. Having the market railroad everyone into using the payment methods they prefer is wrong. I mean, how exactly is that a free market?
It's a free market because anyone can start an exchange. These are not centrally controlled businesses. Exchanges that serve their customers the best will get the greatest reward (that's called "profit" for the entrepreneurally challenged).