It doesn't mean insurers can't charge based on risk, otherwise you totally defeat the insurance principles. It means the policies exist, but it still may be beyond the financial ability to pay. The government will eventually be forced to limit coverage through denial of procedures or care based on age most likely.
If more companies are offering more plans in the exchanges, it is because they believe they can make more profit than if they stay out of the exchange.
The govt. is not setting rates, nor offering coverage. The govt. has set the standard for certain terms of the policy like "must insure", "non-cancelable", and "no lifetime limit". It is the ins. company's responsibility to analyze the actuarial impact of those terms, find out what the projected cost of that coverage is, and set a monthly premium price that will allow them to pay their claims and make a profit for their shareholders. Apparently they can do that, since we are seeing greater participation in the exchange, per my post above.