This is an interesting point. In the long run, this IRS opinion that miners need to report income when they mine, is frankly absurd, and will definitely be reversed in a tax court, if they don't revise it before then.
How do you figure? The network is paying them for their services of mining. Seems correct to me and it is the ruling I expected and predicted since long-time ago.
I can freely produce a work with a market value, and the act of production does not result in a taxable event, in every precedent case. Selling the work produces a taxable event. A network cannot pay you anymore than a gold mine can pay you. You book a profit when you sell the gold you mined. Your paint box does not pay you when you paint a picture. When you sell the picture, a taxable event occurs.