And while it's easy to chalk this up to lazy writing (because that's what it usually is when something like this shows up in a "time travel" story), I'm pretty sure in this case the apparent paradox is intentional.

Our conventional view of causality is that every event has a family of events in its "past" and a family of events in its "future", and that those two families of events can't overlap. But in general relativity the structure of spacetime is itself a mathematical solution of a family of field equations, and there are some solutions to the field equations where this simple view of causality no longer holds. In particular, in general relativity there are spacetimes where in some regions time "loops back on itself" in such a way that a person could constantly march toward the future (as we all do), but yet end up landing in what would have originally been called "the past." On the surface of the Earth, if you travel due East, then you'll eventually come back to where you started, moving in from the West. This looping of time back upon itself in general relativity is the same sort of thing.

In short, general relativity doesn't require "causes" to precede "effects" in any global sense, as in conventional experience. All general relativity requires is that the overall history of events must be self-consistent. The wormhole is there because the humans of the future placed it there, and they were able to survive long enough to construct that technology because the wormhole was there to give them a means to survive. It's all totally self-consistent as long as you don't ask "what would have happened if the humans *didn't* survive long enough to build the wormhole?" The answer to that question would be: the humans wouldn't have survived, a different but again totally self-consistent history of events.

What general relativity does NOT allow (as far as anyone can tell) is the development of a history of events that is not internally self-consistent. For example, the old paradox of someone going back in time and killing their grandparents before their parents were born. Such a scenario wouldn't make sense mathematically in conventional general relativity, and there are conjectures that the fundamental laws of physics (whatever they are) might "find ways" of prohibiting such scenarios. Some people even talk about it as fancifully as "mysterious forces never encountered in ordinary experience intercede to stop the gun from firing", or "the basic random processes at the foundation of quantum mechanics become altered in such a way that the thoughts inside the assasin's head become altered to force him into a self-consistent set of actions." It's all still very mysterious and tied up in a series of mathematical conjectures, the most famous of which is known as the "chronology protection conjecture."

Anyway, as I said, in this case I think it's intentional that they brought this paradox into things. A lot of people don't realize that even though he didn't write the script, this was Kip Thorne's movie before it was anyone else's. The vision from the beginning was for the movie to give representations of interesting issues in general relativity that are accurate (so somebody actually sat down and calculated all those time dilation factors on the planets, for example), and for which issues in general relativity are central to the plot (wormholes, time dilation, etc.). These subtleties with causality are definitely interesting issues in general relativity theory, and much of the most important work on the subject has been done by Kip Thorne.

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.61.1446
(the paper itself is behind a paywall, but if anyone insists on attempting to read it, it can be found by googling the title)