Some of those millions undoubtedly will find health insurance once the state-level exchanges are set up, but there's nothing to suggest that a large majority of them necessarily will, even though the law supposedly compels them to do so.
Those who do won't face a penalty, of course. Some of those uninsured certainly live in states that so far have refused to set up an exchange, but certainly not the majority. It makes sense to give those people an exemption.
Those who would also be exempt include taxpayers with incomes below the filing threshold and members of Indian tribes, for example. Those are worthy exemptions.
Other exemptions, to put things kindly, are more dubious, but still sound reasonable.
They include those who cannot afford coverage, people who qualify for hardship exemptions, individuals who have short coverage gaps, those who don't want to purchase health insurance for religious reasons, members of "health sharing ministries," and individuals who are incarcerated.
The problem seems to be that some of the exemptions (who's judging what's affordable, for example) are so wide that pretty much anyone could qualify for at least one of them. But what about the 2% who still don't? Will they be responsible for paying a penalty? Well technically, yes, but who's counting?