Obviously Assange (et al.) didn't ask anyone if they could leak sensitive data -- and perhaps they should have (but how exactly would that have gone do you expect?) -- but I believe Assange is acting on this above principle.
In summary, he treats government as a conspiracy (liberally defined to be a social network) whose total conspiratorial power (the sum of the weights of the edges of the social graph, where the nodes represent people and the edges the potential to share information) can be kept in check by increasing the cost of exchanging information -- which is exactly the effect of leaking classified material.
In fact, Assange has written down the principles he bases Wikileaks on: https://zunguzungu.wordpress.com/2010/11/29/julian-assange-a...
In summary, he treats government as a conspiracy (liberally defined to be a social network) whose total conspiratorial power (the sum of the weights of the edges of the social graph, where the nodes represent people and the edges the potential to share information) can be kept in check by increasing the cost of exchanging information -- which is exactly the effect of leaking classified material.