That's the crux of most of these discussions regarding medically necessary liquids - TSOs will often question travelers on their medical conditions, medicinal needs, and travel itinerary, in order to make a judgement on whether the liquid is medically necessary. This judgement should never be made by a TSO, because as you said, it amounts to practicing medicine without a license, particularly in cases where the TSOs ignorant opinion of what constitutes medically necessary directly contradicts an actual physician's instructions.

To be fair to the TSOs, though, most of them aren't trying to practice medicine without a license. They're actually trying to practice law enforcement without a license.