The economic pressure will be towards lower markups and ultimately discounts. It just takes time to move towards the competitive equilibrium.

Right now, few merchants accept bitcoin, which let's the ones who do charge a premium. Given bitcoin's lower transaction costs, this is a nice profit opportunity for them. Other things equal, this makes them more profitable than their competitors.

Eventually, competitors notice this and begin accepting bitcoin too, and at lower markups. This drives markups down to the level that's consistent with the relative costs of handling fiat vs bitcoin.