There is a rich set of literature in economics on two sided markets like currency exchanges are. It also contains a canonical model on how to set prices in these kind of markets. The details are very complicated and math-heavy. But the general idea is that the fees for each side of the market -- here buyer and seller or maker and taker -- should depend on the semi-price elasticity of the other side of the market. The reason is a what economists call adjacent complimentarity.

In human words: it is (mathematically) optimal for an exchange to charge different rates for makers and takers, since they react differently to prices. However, the  balance between the fees for each side is crucially important and in reality not easy to determine. If the gap is too wide that is worse than an identical rate for every side of the market.

edit: If you define the market price as the average between best sell and best buy offer, then the taker receives a worse price already. So this has to be kept in mind, when thinking about maker-taker fees. For example, in high volume markets with a low gap between best buy and best sell offer, the maker-taker fee difference should be greater than in a market with low volume (where there difference might even be negative, that is the taker having to pay less fees than the maker).