This should be happening outside of the exchange as well, since pre-existing conditions must be covered. Uncle Sugar is paying most of the tab for the exchange premiums, so the Ins. companies are getting paid.
Some of the reported problems with the exchanges is that is easy to manufacture a qualifying event when one gets sick. People have been dropping coverage in Nov. and Dec because there is no penalty for going 2 months without coverage. People haven't been signing up in the numbers that they thought except for the sick..