What is the usual punishment for adultery? Why is Hester's punishment less severe?

From the text:

“It behooves him well, if he be still in life,” responded the townsman. “Now, good Sir, our Massachusetts magistracy, bethinking themselves that this woman is youthful and fair, and doubtless was strongly tempted to her fall,—and that, moreover, as is most likely, her husband may be at the bottom of the sea,—they have not been bold to put in force the extremity of our righteous law against her. The penalty thereof is death. But in their great mercy and tenderness of heart, they have doomed Mistress Prynne to stand only a space of three hours on the platform of the pillory, and then and thereafter, for the remainder of her natural life, to wear a mark of shame upon her bosom.”