"Don't penalize the rest of the defenders?" What are you talking about? It doesn't penalize anyone, unless they tackle someone in a specific, completely unnecessary and highly dangerous way. If, as you say, it's an isolated case and Roy Williams is the only one doing it (your words, not mine), then nobody else is getting penalized for it, so your point is completely moot.
It's informally called the Roy Williams rule because Roy Williams does it all the time, and is the most prominent offender. Rules are oftentimes nicknamed to this effect: it says nothing of how prominent it is or is not. Bottom line: horse-collar tackling is unnecessary and highly likely to cause devastating damage to the player being tackled. It fits every definition of cheap and dirty, and its only advantage is that it allows you to drag a guy down after he's utterly beaten you. It ends careers, and there's absolutely no need for it.
