well the US is predicated on the principle that the power of the government is derived from the people, so in principle (and ultimately de facto) the people of the US have more power than the government, and telling on the abuses of power is a means to get the problem rectified (or else why would Manning have done it?)

There are a lot of reasons to release hundreds of thousands of documents at random that have nothing to do with rectifying problems. Hurting an organization that doesn't support you, for example.