The information is public domain by definition because neither Wikileaks or the US government claim copyright on it. The government usually can't prosecute the general public for distributing classified information they get from a government employee, a web site, or a newspaper. They can try to get an injunction against publication, but once it has been published the public can generally use the information freely. In this case, there has been no discussion about prosecuting the new york times or even an injunction to prevent publication of the leaked documents. In the case of the pentagon papers, the government only prosecuted the employee who leaked the information, not any of the newspapers that published the leak.
I agree that they shouldn't have published the cables about how our diplomats think that certain foreign leaders are dumb because it didn't provide any useful information. The cables about Iran do provide useful information about how the middle eastern governments ignore the will of their own people. Bombing Iran is very unpopular in the countries whose leaders were secretly advocating for an attack on Iran.