> My question is - why should they be taxed even more than the rest of us?
Among other reasons: because it is government and the legal system which makes amassing and maintaining large fortunes possible; being poor without organized society is easy to achieve, being rich is a possible only with the order imposed on society through law. Therefore, it makes sense for the rich to be taxed not merely proportionately to their wealth, but disproportionately so.
You can be rich in the absence of law and organized society, too. You just have to be rich enough to hire/inspire the goons to keep the peasants cowed.
What you said is "you can be rich without law and organized society, if you can manage to establish yourself as the government".
Which is true in a sense -- though then you've organized society around your interest, and established yourself as the arbiter of law -- but not really an argument against the rich bearing disproportionately the cost for government.
Among other reasons: because it is government and the legal system which makes amassing and maintaining large fortunes possible; being poor without organized society is easy to achieve, being rich is a possible only with the order imposed on society through law. Therefore, it makes sense for the rich to be taxed not merely proportionately to their wealth, but disproportionately so.