I'd have to disagree, we had multiple presidents who were drug users; the hippies are in power now, and the war on drugs marches on.
Using drugs is simply another thing that the ruling class openly admits to doing and doesn't suffer notably for, while they throw the book at any other citizen for doing the same, or less.
e.g. dodging taxes, soliciting prostitutes, cheating on their spouses, not paying child support, hiring illegal immigrants and maybe even covering up the odd mysterious death.
I don't know the specifics but I strongly suspect that coming out against the war on drugs will lose them more support than it would gain them. Support in terms of some mix of votes, finance, influence and connections.
If this was un-true then some politician would have realised that standing for legalisation would win them an election and would have done so by now.