Originally Posted by Konda

The test is not just for alcohol, it is a test to show if you are fit to drive or not.

It's called DIU, Driving under the Influence. It doesn't have to be alcohol, it can also be under the influence of something else (like drugs).

If you can't do those simple tasks you are very likely to be under the influence of something and you are not in a shape to be driving.

If he only did a breathalyzer it would have shown that he was not drunk, but that doesn't mean he wasn't under the influence of something else. He was obviously not fit to drive due to the medication he took, so the test does make sense as it proofs that he couldn't perform the tests, so shouldn't have been driving.

There are lots of things you can be under the influence of that they can't test for, or at least not with instant results. By doing these tests they can proof in court that the person was unfit to drive.