True tax reform is predicated on the principle that all income should be taxed at a low rate once, and only once. But much of the wealth that Mr. Soros spent years moving into his Open Society Foundations will never be taxed. A gift of billions of dollars of appreciated stock escapes any capital gains tax, and the estate tax as well. So Mr. Soros can donate appreciated stock that Open Society Foundations can liquidate without the government ever taking a cut.There’s more. When a person donates untaxed, appreciated assets to a private foundation, he may also deduct up to 20% of its market value on his personal return, carrying forward this deduction for five years. This double write-off may be the sweetest deal in the tax code.The donors also can retain control of the money within the private foundation for years or even decades before it is disbursed. Since the foundation can employ family members at six-figure salaries for life to “administer” it, the umbilical cord to the donor never has to be cut.

Congress should stop ignoring this tax-avoidance scheme. The super rich have already poured hundreds of billions into private foundations, but the figure could soon be in the trillions. Mark Zuckerberg has pledged to give away 99% of his Facebook shares, currently estimated to be worth somewhere around $70 billion, and much of it will go to a foundation his family controls. Bill Gates and Warren Buffett have each put roughly $30 billion tax-free into the Bill and Melinda Gates Foundation. This has left the foundation so flush that it spent $500 million on a 12-acre, 900,000-square-foot office complex in Seattle for its 1,500 employees. This is philanthropy?

I don’t question these billionaires’ right to do with their money as they wish. I’m simply arguing that Congress shouldn’t let the rich and politically powerful use private foundations to escape taxation. This loophole is one reason for an anomaly in our otherwise progressive tax code: The top 1% of earners pay an effective tax rate of 23%, but the top 0.001% pay only 18%.

Mr. Buffett has sanctimoniously denounced the fact that he pays a lower effective tax rate than his secretary. His suggestion is that Congress raise taxes on capital gains. But even if the tax rates were lifted, say, to 50%, Mr. Buffett still wouldn’t have to pay it on the tens of billions of dollars he puts into private foundations, and he would still be able to deduct a fifth of that contribution on future tax returns.

This tax favoritism might be defensible to promote genuine philanthropic activities. Many billionaires, such as the Gateses and David Koch, have heroically donated to fight cancer and malaria or provide relief to hurricane and earthquake victims.

But others, including Mr. Soros and Michael Bloomberg, have turned private foundations into massive de facto lobbying operations for bigger government and liberal causes like higher minimum wages, gun control, universal health care, and a carbon tax. Mr. Soros’s $18 billion gift alone is the equivalent of maybe 100 Heritage Foundations. This kind of weaponized philanthropy has the potential to undermine the American free enterprise system.

Yes, billions go to groups on the right, too, from Mr. Koch and others. But regardless of ideology, why shouldn’t tax be collected before the money is given away? What message does it send that the Republican tax-reform bills retain this trillion-dollar loophole for the super rich, at the same time as the House plan eliminates the adoption credit for middle-class families who want to help children?

One simple solution would be for Congress to apply the capital-gains tax to assets of more than $1 million before they are transferred to a charity. This could even finance cutting the capital-gains rate to 15% for everyone.

Alternatively (or perhaps in addition) Congress could cap deductions for any given household to $250,000 a year. Under this kind of plan, Mr. Soros would be able to write off only a tiny fraction of his multi-billion dollar gift.

This isn’t an argument against charity. But selfless and effective giving is not motivated by tax breaks. Two-thirds of Americans don’t itemize their deductions, yet millions give until it hurts. In the 1980s, individual donations to charities surged, even as the top tax rate—and thus the maximum value of the write-off—fell from 70% to 28%.

The question is whether a tax code that encourages dynastic family foundations is good for America. If Congress stopped letting billionaires pour money tax free into the foundation-industrial complex, it would go a long way toward lowering rates and making the tax code fairer for everyone. This would help the economy grow faster, which is the best way to help those in need.