I wouldn't necessarily say tax evasion, but more tax diversion. The wealth use charity to try to sell the idea that its better to let the wealthy have discretion over how they use their power to make society better. People look at the huge numbers and think the wealthy are being extremely charitable, but as was pointed out earlier, people in general really don't understand big numbers. If looked at as a percentage, most of the wealthy are really giving a very small percentage to charity. Additionally, charity is often also used as a means of exerting power and influence.
The NYT had a great opinion article written on this, and linked is a video of an interview with the author also discussing the problem.