The other hand (Score 1)

Having worked for 12 years for, ah, shall we say the largest car company in the World, and also having been a lead on that companies' OBD-II development, I can say with confidence what one of the main motivations for keeping the OEM bus codes a secret is this case:

It really is a case of a few bad apples ruining it for all the honest shops, backyard mechanics, and hotrodders out there. What happens all too often is somebody adds/modifies/improves some feature on the car, usually with very little knowledge of how the thing works, and then has a problem - like breaks or burns out some part. Then they remove their mod, and come into the dealership complaining about "doesn't run right" - hoping to be taken for an innocent fool. The repair shop usually can spot what the most likely case is, but goes ahead and turns in a warranty repair anyway - the manufacturer ends up paying for it...

This is real $$ here - and anything, even a diagnostic display that says "turbo overboost", that can help/encourage/egg-on the few bad apples is not released (If anybody out there rememebers the '86 Buick Riv/Olds Toronado with the CRTs, did you know that with a special key press the CRT became a built in real time dianostic/test station? Things like running parameters, test output, etc. were available... just not publicized). BTW, the original reason for standardizing on CARB's OBD stuff was so that small shops could buy 1 $200 tool and work on the emissions of all vehicle manufacturers. Well, today, its more like a $700 tool..

The case that all manufacturer fear, is that with new car systems, substantial control of the vehicles basic safety parameters are at the command of the electronic systems onboard - nobody wants to see a person modify their own car, and end up having an accident that kills them (like 200 degrees of timing adjustment available to the engine computer, Yaw rate selectable by the Dynamic Stability Control, etc.).