While smoking is legal, it is still an unhealthy activity to partake in. The fact is, many people view it as a filthy habit. So, if you are wearing your letters and you light up, would you want to risk that the public would see your letters, form their own negative preconceived notions about your chapter based on what they have seen. Yes, it is a bit hypocritical to "smoke behind closed doors", but life isn't perfect.