No, of course they don't specifically give out information about driving safely while drunk -- However, there is a legal limit beyond which the government has decided that it is unsafe to drive, and therefore has legislated against allowing people to drive drunk, rather than suggest safer ways to go about it. The analogy for injecting in the case of this argument is your own - however you redefine legal as non-stupid, and illegal as stupid - by your thought process it is a 'legal' (non-stupid) thing to inject pure injectible solutions and pure drugs, but it is 'illegal' (or stupid) to inject impure drugs (this definition should, in practice, be extended to include certain pure pharmaceutical products also which are hazardous to inject, such as crushed tablets, normison gel capsules, etc.).
Again, I reiterate that both of these approaches are helpful, but if someone wants to go to excess in the case of drinking before driving, or to excess in the case of injecting street drugs (regardless of the impurities) then neither of the above approaches helps to make that decision safer for the person, or the people around them. The message is obscured in the delivery, because the person switches off instantly, before even beginning to consider the reasons WHY the message was phrased in such a way. Since alcohol is legal, and there is a legal limit, the government can't be seen to show any leeway on that limit. Otherwise it's like saying "Don't break the law, but if you do, be careful not to hurt yourself". However, we don't enforce the non-stupidity part, because it's everyone's right to make their own mistakes or refuse to learn from the mistakes of others. What comes with exercising that right is a set of circumstances or outcomes, which aren't always apparent to the person when making their decision.
While it is legal, sensible, safer and non-stupid to avoid driving drunk and shooting up dirty street drugs, some people WILL still undertake these actions regardless of their stupidity or circumstances at the time. Quite often judgement can be impaired through existing drug use or withdrawal, mitigating circumstances, or simply lack of forethought or appreciation of the consequences. Impaired judgement doesn't necessarily imply stupid, however, and I think that taking the time to explain the consequences, and ways to circumvent them, helps people to get a better vantage point from which to make the decision, which is ultimately their own to make.
BigTrancer