> Actually the US has tried to keep its intelligence operations secret, as do practically all nations.
There's a fine line between keeping the details of your intelligence operations under wrap, something all countries aspire to (and the US, possibly by virtue of being in everyone's crosshair, is not particularly good at), and denying the legality, or even the will, to conduct such operations. Time and again, US judicial authorities, all the way up to SCOTUS, have upheld that the protections afforded by the Constitution do not apply to foreigners (with some very tiny exceptions) and that the US are free to do whatever they want in that regard (not adhering to many other international treaties that may put them at odds with international laws).
"World opinion" is a completely unmeasurable concept (and ridiculously narrow-minded: as if "Rest of the World" was just a comparable entity to "US"), and at any rate, this type of "opinion poll" is laughably pointless in how easily it can be biased in whatever direction the pollster wants ("Do you think the organisation headed by alleged rapist Julian Assange is doing the work of God" vs. "Should the US be allowed to send a drone strike after an Australian citizen residing in Europe?")... But it is plain obvious that non-US citizens only care about US national interest in a very tangential way (and considerably less so, in our days where there is no clear opposite block to be afraid of), and therefore are much more likely to be, at worst, neutral toward the release of US diplomatic secrets (and the people doing the release).
Most importantly, your original point apparently failed to realise that the informant is a citizen of Iceland, where popular opinion does not favour the US gvt against Wikileaks, to say the least.