That's simply not true. Firstly, people aren't rational. See this list of "cognitive biases" as an indication: https://en.m.wikipedia.org/wiki/List_of_cognitive_biases

Secondly, there are all sorts of drugs, some addicting, some not addicting, some legal in some places, some illegal in other places, some making people "irrational", some making them more "rational". There's no correllation between these attributes.

Legality depends on historical accidents more than anything. Alcohol is in fact one of the more addicting drugs out there, see the high percentage of "functioning alcoholics" out there.