But is "magically" not enforceable, rendering taxes voluntary, sic.
Hardly. In a real property transaction the change in ownership needs to be recorded with the state. Taxes will be paid by all parties. Bitcoin isn't some magical tax free currency. Anything of sizeable value causes ripples in the real world. Say NewEgg accepted Bitcoins and this year did $50M in sales. Where did the $50M in inventory come from? think it is going to leave a paper trail. NewEgg would wisely pay corporate income tax on all that revenue. If they didn't come audit time it is going to be trivially easy to bust them.
Sure you might be able to sell a thousand dollars in gold coins and avoid taxes if you use Bitcoin but .... you could do the same thing with cash. Anything involving real world inventory or property is going to leave paper trail and tax agencies aren't just going to "miss" billions of dollars in commerce just because it didn't involve legal tender.