Blacksword,
As has been pointed out ad nausum, the procdures are copied from the Road Traffic Acts. The drugs side of it is perfectly well defined, if you impaired by drugs you commit an offence, end of story. Whats undefined about that?
The artical you refer to says lots of things, it does not however say in any way shape or form that there is no link between BAC and alcohol in Urine.
Please have a read of this, it may help you, it refers to the RTA, and as you have been told, that's what this law is based on.
http://www.trl.co.uk/static/dtlr/pdfs/TRL495.pdf
You will note under the heading methodology it says that URINE samples were used to test BAC and for drugs. While this involved fatal accidents the princible is the same, there is a link between BAC and urine alcohol levels.
The BAC and the breath and urine equivilents have been set and whether you, me or anyone agrees is irrelevent, the law exists. As to if its bad, well I can see no reason why its bad per say, except people may not agree with the levels set.
Whats becoming a worry in all of this is that you are putting a very amateur spin on what you have read, and applying your opions. The result of which could get someone into trouble if they are daft enough to give your theory a try. I have heard lots of theories on how to avoid being convicted for drink drive all except one were B*******s. (and that had to do with people who use funny handshakes). So please desist until you have some knowladge and experience in the subject.