Why did the lawyer do 15 years instead of 5 years?

The lawyer committed to fifteen years in order to prove a point.

The death sentence and the life sentence are equally immoral, but if I had to choose between the death penalty and imprisonment for life, I would certainly choose the second. To live anyhow is better than not at all."

A lively discussion arose. The banker, who was younger and more nervous in those days, was suddenly carried away by excitement; he struck the table with his fist and shouted at the young man:

"It's not true! I'll bet you two millions you wouldn't stay in solitary confinement for five years."

"If you mean that in earnest," said the young man, "I'll take the bet, but I would stay not five but fifteen years."