It is unfair because 3 grand to a person earning 15 grand is worth a lot more than 30 grand to someone earing 150k. It puts a disproportionate burden on the lower classes. Consider other taxes too, that everyone pays equally. It would shift the burden to poorer people in society.
Did someone really say a flat rate of tax for everyone ? Blimey, when is the next meeting of 1922 committee ?
Taking the required mean tax average, that would mean the poorest would be handing cash to richest.