Even if wikileaks physically returned whatever they physically received, but made a copy for their own purposes, is that still stealing? There are laws that apply to information, such as copyright, but this information is not protected by copyright. I read somewhere that there was no classified information in this release, so it also wouldn't be protected by those laws.
For me, the 2 main questions are: 1) is it legal (yes) 2) will it do more good than harm? (unknown)
US officials stole classified information from the UN. Chinese officials stole classified information from Google. Let's be honest here: they're all at it.