Actually what you say is not true. Owise tracks your predictive success in each category. If you have built up a good track record in Entertainment that won't give you a high weight in Politics. (In fact, we've found people who are very good in Entertainment are often simply awful in predicting political events, but that's another issue...)
We do give you a small fraction of your "overall" weight if you try your hand in a new category you've never predicted before, because we've found having some track record does correlate a bit across categories, but we're refining how that part of it works as we gain more data.
We are also refining exactly how fine grained we go with this category idea. Right now we track weights by top level category (Politics vs. Weather vs. Entertainment) but in the near future we may go down the heirarchy more deeply if you have enough events within a category (Entertainment/TV vs. Entertianment/Movies for example).
Owise is very much a new, experimental system, very much in the data gathering and refinement stage. I'm not claiming it's perfected yet, but we've thought through quite a few of the issues.