Can someone explain to me why everyone who earns $200,000+ a year in income should be fair game, but some socialite whose trust fund accrues that much or more in capital gains should be off-limits?
You want people to save and invest. They're not putting it under their pillows (else it wouldn't be accruing gains). Instead, that money is being put to use in the economy, enabling people to build factories or start companies.
On a more serious note, if you make that kind of money for any significant amount of time, you're going to be a member of the investment class pretty soon (i.e. making your money from capital gains) unless you have poor money management skills (or unusual circumstances, like supporting 10 people, or an expensive gambling habit, etc.).