Really. Some DUers are actually complaining he gave a large chunk of his income to charity, and the rest are wondering why he didn't hit the 50% of AGI cap on deductions.
And five minutes on google told me that the reason. Basically, when he was appointed, to avoid the charge he could benefit from the exercise of Halliburton stock options, he pledged to donate any profit from the exercise of those options to charity; and the trust that administered the agreement in turn agreed to pay any extra tax liability that arose from the 50% AGI limit.
So the Cheneys just tried to ensure that, if they ended up donating more than 50% of their income to charity as a result of an agreement they entered into to avoid a appearance of conflict of interest, they wouldn't have to pay the government a million bucks or so of the remaining income as a result of giving several millions of dollars to medical research and education.
Takes a special kind of person to criticize someone else for outstanding generosity.