FlapsOne
I'm not quite sure where your're coming from?
<<If he was positioning, he was on duty>> Yes
<<therefore should not have been drunk>> Under the ANO - yes - nobody should be "drunk" on an aircraft.
However, "having a drink" while on duty (e.g. while positioning) is not against BA rules, or any others I know, provided out of uniform and obviously not operating afterwards i.e. a rest period first.
The worrying thing as you say is that this seems to have been noticed by a number of people over a period of time, and all that seems to have been done was lay the trap? If all the above is true, it does not condone the behaviour of the individual, but some direct intervention earlier might have saved the individual and BA a lot of hassle, whilst still getting the point across.