How far should employers go in regulating off hours behavior?

Couldn’t you say the same about skiing, or even driving a car? Both have well-known risks of injury and death to oneself and others, and both are optional activities. According to your logic, people who ski or drive a car are making errors in judgment, and therefore should not carry guns.

Driving is a required part of most forms of field police work, smoking is not.

A local ambulance co near me actually had a policy of not hiring currently employed firefighters due to their propensity for getting hurt and missing work. So dangerous outside activities can be an issue.

True - while on the job. But by driving on his personal time, he’s increasing the risk to himself (and others) beyond what’s required for his job, like a firefighter who enters burning buildings in his spare time.