Insurance stocks plunged when it came out public option was in the bill.
Then spiked when Joe Lieberman announced he would try and kill it.
Think Progress » Insurance Stocks Plunged As Reid Announced Public Option, Spiked After Lieberman Vowed To Filibuster It
The market is not stupid.
They know the public option will lower adminstration costs of health care by eliminating for-profit insurers for millions of Americans on the public plan.