EA is not binding like ED.
Merit aid is a recruitment tool for higher stats. Some schools have an formula that automatically grants merit aid based on stats. But at schools were merit aid is DISCRETIONARY, if the non-FA student applies ED, and is bound to commit anyway, then what’s the point of dangling a non-need-based incentive?
Typically, the people who should be applying ED are those needing FA (they can decline if the FA isn’t enough) and those prepared to be fullpay (they have no basis for declining).