A: They tested you because they can, and because marijuana use is still illegal. Sorry, but even when you are showing that you may have a medical need, but your medical marijuana card isn't issued yet, the court may order you to test "clean" or negative for any other substances (including alcohol) during the period of your probation.
An alcohol and substance abuse evaluation is often part of the sentencing process. It's a report that is later given to the Court and Probation department, and it can be quite influential regarding your sentence.
Let's face it, taking care of your legal problems should be foremost in your mind right now. So take your time with the evaluator, and don't feel like it's an inconvenience. Spending time in jail or on community service are inconvenient as well.
The evaluator may also ask questions about your family life, whether your parents had alcohol or drug problems, and other questions that don't seem particularly relevant to why you were pulled over. Sorry, but that's part of the assessment as well, and gives a clearer picture to the evaluator as to whether you may be at a risk to re-offend. A second evaluation may also be part of the sentencing and probation process, nearer to the end, to show that you're improved over the term of probation.
The test, for which you tested positive, will possibly be re-administered between now and the end of your probation. Today's test will be used as a "baseline" for the THC that's in your system, and as long as you're ordered to test, that level needs to go down in your blood, otherwise you may be facing a violation of your probation - no laughing matter. It means that the underlying sentence from the drunk driving charge may actually happen to you, even if it was pleaded out in court.