From Martin Feldstein:
A key feature of the House and Senate health bills would prevent insurance companies from denying coverage to anyone with preexisting conditions. The new coverage would start immediately, and the premium could not reflect the individual’s health condition.
This well-intentioned feature would provide a strong incentive for someone who is healthy to drop his or her health insurance, saving the substantial premium costs. After all, if serious illness hit this person or a family member, he could immediately obtain coverage. As healthy individuals decline coverage in this way, insurance companies would come to have a sicker population. The higher cost of insuring that group would force insurers to raise their premiums. (Separate accident policies might develop to deal with the risk of high-cost care after accidents when there is insufficient time to buy insurance.)
In an attempt to prevent this, the draft legislation provides penalties for individuals who choose not to buy insurance and for employers that do not offer health insurance. But the levels of these fines are generally too low to cause a rational individual to insure.