Because it's an interesting example of tax laws being, for lack of a better word, wonky.

Tax should be on profit, not revenue or turnover. By offering lower fees, Vanguard transfers wealth from the investment fund industry to the consumers of its products.

I see no difference between what Vanguard is doing here, and what WhatsApp did for instant messaging - disrupting an incumbent industry with a new and cheap option.

I don't see how anything about it is wonky. Care to explain?

The difference is that Vanguard is owned by it's customers so the low fees are transferring wealth from the consumers back to the consumers while (allegedly illegally) lowering their tax bill.

It's the ownership structure that makes things different.