There interested in not gathering data that would cause someone to admit criminal behavior.

As far as I'm aware - and correct me if I'm wrong - drug use is not a crime (and by extension admitting past drug use isn't either). Possession, operating a vehicle under the influence, etc, are all crimes, but actually having used drugs isn't a criminal act.

There also the issue of possible outsiders being able to say: "30% of LW participants are criminals!"

The current survey (hell, the IQ section alone) gives them more ammunition than they could possibly expend, I feel.