Of course they can "afford" it. But so what? Every single billionaire can "afford" to be taxed at a 99% rate. The bigger question: is it in any way fair, just, or moral to tax society's most successful more heavily just for being more successful? EDIT: The other salient question that I forgot to mention is: "why do we need more taxes?" Our government already can't manage what they have with any sort of reasonable eff…
http://ingrimayne.com/econ/resouceProblems/Tournament.html
The differences between individuals (say top 10,000) in certain skills becomes negligible due to the sheer amount of people in the world. However, due to winner take all systems, it is possible for someone that works 0.01% harder or luckier to take 100% of the reward. This is why it is possible for wage disparities to increase beyond ratios of 1000 to 1 without someone having to work 1000 times harder.
The reasoning behind increasing the tax rate on the super wealthy is for several reasons
1) It is easier to accumulate money the wealthier you are. This creates a negative incentive for the super rich to work as hard since it becomes easier to simply buy out competition and form monopoly status. Or crush everyone else due to economies of scale rather than true innovation.
2) This conundrum is well known in many companies where they try to pay their employees as much as they can, but not too much, or else they retire early. If you believe in the mantra that the richest people are the most productive, then it is obviously in society's best interest to keep them productive.
3) The more percentage of total wealth you have, the more US infrastructure you use and rely on. For example: Exxon heavily relies on the US military and government sponsored roads. Tech companies like Google and Microsoft heavily relies on the nationalized electric grid, subsidized education, the invention of the Internet, and subsidized telecoms that allowed people to use it. The average Joe does not have as much to lose if these tax subsidized services were to disappear tomorrow.
4) The tournament theory I mentioned above. The vast differences in wealth are due to situation or luck, and not because someone truly works 1000 times harder than you.