Pretty simple, actually - The NSA already collects ALL data flows over the internet. First, they follow your public key - on the publicly available blockchain, match it up with your personal internet MAC address (cutting straight through your VPN with the already existing back-doors), match it up with your mailing address, send the info. to the IRS, and the IRS withholds your profits from your next tax returns, or shows up on your doorstep to do an audit, or sends the cops to arrest you for tax fraud. It has actually already been done many times, and it will become more and more common as people make profits in cryptos - because taxes are as sure as death.