Telling people to quit destructive health habits is most definitely in the purview of the physician
You're a mechanic, highly paid and educated, but still a mechanic.
Someone comes to you with a problem, you're professionally obligated to tell them what caused the problem. And if you can't fix it because of some action on their part, that's their problem. You've discharged your duty.
Your injecting your religious or personal philosophy into what should an arm's length, professional and science-based relationship diminishes your authority.
I see . . . you have a problem with me telling people that smoking is bad for them at that they should quit as soon as possible. That's not "injecting anything" it's stating the truth.