De dure, not de facto. Show me a report of someone getting arrested for it. Hence my accurate description of his ridiculous scenario appealing to a slippery-slope style train of thought. Case closed.

It's still not a slippery slope. It's an analogy that illustrates what would be possible, de jure. Honestly, there's no slope involved at all.

It is not an analogy. What is the analogue? The slope is descending from now (you cannot get arrested for singing happy birthday) to then (you can). Slippery.