I agree it's probably a strategic error, but I don't think it's actually very easy to distinguish governmental secrets from corporate ones, especially if you have to be somewhat opportunist in what secrets come across your desk. For example, what if you were interested in exposing the web of corruption that is the African oil and gas industry? Leaking documents on the subject from major multinational oil companies is probably the best route. You could try to only go after it from the governmental side, leaking documents from corrupt regimes that take bribes and rig auctions, but if you're a mainly western-based organization like Wikileaks, you might have easier access to internal documents from Exxon versus internal documents from the Nigerian government. The Nigerian government may have also been more careful about whether any of that ever got written down, so Exxon's side might actually have the best documentation.