Most professions have ethics rules and you WILL lose your right to practice your profession if you violate them. They include Marijuana use, DUI, domestic violence, bad credit. These people lost a decade spent learning their trade, the same decade the NFLers made $100+million.
If anything the NFL is EXTREMELY leniant, in that it is very difficult to ever lose your right to play without a multitude of crimes or instances. A doctor or cop accused of wrongdoing would immediately be put on leave WITHOUT a trial. ANY convictions for these individuals and they can lose their job FOREVER.
When was the last time an NFL player was banned from the NFL for life for a single conviction of DUI?