...because delta knows that if they didn't have that pay then they would unionize and probably be able to negotiate something as good if not better (edit: better than what they want to offer down the road. They beat the union rates so they can point at it and say "I told you so!", discouraging a would-be unionized workforce now so they don't have to negotiate with a union later and can get away with the sort of underhanded shit that a strong union would be able to stand up against, including but not limited to lower wages and poor working conditions.)

Just because there isn't currently a union doesn't mean a corporation like that isn't kept in check by both the realized and potential presence of unions in their industry...