Sales taxes are considered regressive taxes because the disproportionately affect the poor and lower classes of society. Lower classes spend a much higher percentage of their income (in fact, almost all of it) than middle and upper classes, and are therefore are subjected to a greater percentage of taxation. This is based on the percentage of income being taxed.
I do not have any figures, but it is possible that middle and upper classes pay just as much, if not more, total money in sales taxes. You would have to look up the figures on that...