DD has a defined limit, because without it, how would you draw the line? What would be considered dink driving and what wouldn't? Whereas theft... Theft is theft. You can drink a bit of alcohol and be perfectly fine (in fact some studies have even suggested a slight boost in reaction times, whether that's down to alcohol or not is another matter). You can't really steal and not steal if you get what I'm saying... Maybe taking more free samples than you're supposed to I guess (though I wouldn't exactly call that stealing)?
Also, I'm sure the case of "feeling fine" has been made before... You might feel fine, but you actions may not reflect that. Feeling fine and being fine are two different things. Although, I don't think police should bother with people who are slightly over the limit, write them up a caution (2 cautions = Dink driving offence) and let them go on their way, it's a waste of time and money taking them back to the station if they're basically guaranteed to blow under the limit by the time they get there (roadside breathalysers can't be used as court evidence as far as I'm aware).
As you said, I wouldn't support a 0 mg alcohol limit, because it isn't practical... To even suggest that someone is unfit to drive after drinking as little as a can of shandy is downright idiotic...