Remember he's playing an iterated game. So, if we assume that right now he has very little information about which area is the most important to invest in or which areas are most likely to produce the best return, playing a wider distribution in order to gain information in order maximize the utility of later rounds of donations/investments seems rational.
I remember reading, on the topic of optimal charity, that it's only rational to select a single cause to donate to... until the point of giving enough money to noticeably change the marginal utility of each additional dollar. (Thiel has that much money, of course.) This information-gathering strategy could be a new reason for spreading donations at the level of large-scale donations, if it hasn't been discussed before.
http://blakemasters.tumblr.com/post/24464587112/peter-thiels-cs183-startup-class-17-deep-thought
Some perspectives on AI risk that might be interesting. Peter is (the primary?) donor for SI, and an investor in AGI startup Vicarious Systems.