I don't expect anyone to be 'saints' and I am perfectly fine with social and light recreational substance use but that is world's away from drinking fifth's of vodka or banging grams of heroin which is unprofessional and unacceptable from someone whose hands you entrust to life in. Physicians have ethical responsibilities and just like how it is unethical for them to operate on family members because they couldn't be objective, it's not right for them to engage in behaviors that could adversely affect their judgment which substance abuse (beyond light, social use) undoubtedly could.
You can argue the relative detriment of individual substances all day but my point is that *beyond* light, social use the doctor is taking risks that are untenable for their profession.