It's all about the sham John & Cindy McCain foundation:
Between 2001 and 2006, McCain contributed roughly $950,000 to the foundation. That accounted for all of its listed income other than for $100 that came from an anonymous donor. During that same period, the McCain foundation made contributions of roughly $1.6 million. More than $500,000 went to his kids’ private schools, most of which was donated when his children were attending those institutions. So McCain apparently received major tax deductions for supporting elite schools attended by his children.