I want to know, what else does the government want to regulate the cryptocurrency blockchain for if the FBI can perform such a rigorous search on all the virtual accounts that have been created online by the couple? And found the private key linked to the wallet that received the funds, and they recovered over $3.5 billion and recorded it as the biggest seizure in the cryptocurrency niche.
You can't expect the government to buy into a complete decentralized technology and one that can't link a particular address to a particular individual except by monitoring its consisyent transactions across exchanges.
I think the transaction the Government cannot track in the block-chain technology has not be made. FBI claim that they will endeavor to trace any fraudulent activity in the cryptocurreny network through their 21st century investigative technique. (I don't know what they mean by that)
The blockchain in the decentralized network still covers your full privacy but once associated with an exchange the privacy is lost. The FBI was able to carry out rgeir search because the couple had a link with bitfinex which is very much a regulated exchange so doing their tracking might seem much easier