ordain that the income of any government official may not exceed 150% that of the average income in their jurisdiction.

The incomes of federal and executive government officials has exceeded the reasonable amount.* Therefore, it must be regulated. So, by saying that it cannot exceed one and a half times the average income of their governmental jurisdiction, they will be more motivated to do something to improve the conditions of the area, thus raising the average income.
*This law would include all government (local, state, federal, etc.), not only federal and executive.