Reliance on sales tax means than the people who pay the highest percentage of their money in taxes are those whose money gets spent - all or almost all of it, no savings - and those who do not have enough money to spend it or save it outside of that jurisdiction. So sales taxes disproportionately hurt the poor.

As for robbing the rich to pay for social programs: if those social programs aren't there, the poor still need to eat, so they generally turn to crime, and guess who they're stealing from? It's cheaper to pay for subsidized housing, food stamps, and welfare, than it is to cut that person off and make them live on charity or have them turn to crime.