It's enforced when you yourself declare how much electricity your mining operation used, because you want to expense that cost to avoid paying unnecessary income taxes. This isn't about personal activity, and frankly this reflex to argue against regulation of commercial entities as if they're aimed at individuals is odd. If you do your own individual-scale cryptocurrency mining and spend the resulting coins for your own personal uses, nobody is going to come knocking or really even care post facto - because it's obviously quite unenforceable at an individual level. What this really affects is industrial scale operations that report profits as fully system legible financial income.