It is NOT their money, just like your tax dollars are not YOUR money. They are our society's monies. Progressive taxes maximize revenue in the most just way, with minimal risk of non-compliance. Those who have benefited the most under our system rightfully pay the most.
The ability to pay is also important, as there is a direct connection between increasing earnings and decreasing percentage of savings. EVERY Econ 101 course discusses the consumption vs. savings ratio. Since the wealthy apt to simply save more, it is of economic benefit to tax them higher and redistribute to people who will directly spend it.