David Rubenstein, a co-founder of buyout giant Carlyle Group, chalked up another deal on Tuesday. This time, it wasn’t Carlyle purchasing Dunkin’ Brands, or Manor Care, or an under-performing industrial asset.

Rubenstein bought a copy of the Magna Carta. Yes, the Magna Carta. A rare 710-year-old copy of the document was sold to Rubenstein on Tuesday at Sotheby’s for $21.3 million. That ought to go over really well with the SEIU and other union and activist groups accusing Rubenstein and his buyout brethren of making too much money.

Rubenstein said that he bought the document because he wanted it to stay in the United States. As a former member of the Carter administration, Rubenstein said he viewed the purchase as a sort of favor to the U.S. government, which has been generous to him.

If you follow private equity’s tax structure, indeed, the U.S. government has been kind to Rubenstein and his peers. Debate rages on about the pros and cons of carried interest, but the bottom line is that buyout firms are charged 15 percent for their profits, as opposed to the 35 percent ordinary income rate.

So even if you think Rubenstein’s tax bill is unfair, you can’t say he’s unpatriotic.

(Photo. David Rubenstein, Reuters file)