This sounds a lot like one of those correlation =/= causation problems..
The study doesn't take into account whether a disease, deficiency, or genetic disorder may be the reason why a person is taking the supplements, and that it might not be Omega-3 itself that is the cause.
I'm currently taking a fairly large dose of Fish oil under direction of my Dr to help with my triglyceride levels because I can't tolerate niacin. Like i said above, if I get prostate problems down the road, there are plenty of other factors at play, not just omega-3 supplements.