You couldn't keep Hales in the England squad, essentially it would be the ECB condoning recreational drug use when the England players are role models for so many kids. BBC reporting apparently it happened last year, but Hales was given assurances that his compliance would not cost him his world cup place.
the ECB "insisted on Alex taking certain rehabilitation measures following his suspension" and that, "at every stage, Alex fulfilled his obligations and both he and his representatives were given assurances that any suspension - again under the ECB's guidelines - could not affect his selection for the World Cup"