To put it a little differently, "Friends don't let friends drive drunk" was an Ad campaign led in joint by the Ad Council, and the US department of transportation. it was later used by the D.A.R.E campaign as part of their propaganda (good or bad, that's what it is)
The effectiveness of this campaign is that it calls your loyalty into question. If you do allow your friend to drive drunk, you are, in the ad campaign's opinion,not their friend. The problem with this is that rather than telling you why it's actually irresponsible and wrong, they simply attack your integrity as a friend. That's not very convincing, and in some cases, may encourage the behavior out of juvenile spite. (in my personal experience, one who allows a friend to drink and drive doesn't give a shit about his personal integrity)
The slogan is one of few to last through time. Today, the phrase is still used by D.A.R.E and spouted by driver's ED. teachers all around the U.S of A!