Because the data is there _forever_ and there's not really anything they can do about it, which stands in stark contrast to how they're used to operating.

There is no person they can throw in jail, no corporation that can be sued, no servers that can be seized, domains taken over, etc, that will result in that data no longer being available to those that know where to look for it.

The data just remains online until such a time that the coins of a given chain are worth $0.0 and coin holders no longer have a financial incentive to keep the ledger online.

FWIW the ones I talked to where more of the door kicking variety, not the 'cyber' type.

Are you asking why it freaked out real FBI agents or the ones in lozaning's imagination?

The FBI doesn't give a single damn hoot about "data exfiltration" via blockchain metadata. Anyone with half an understanding of any of those terms knows what an absurd implication that is.

For concerns about "data exfiltration", discussing blockchain doesn't even make a single damn bit of sense. Blockchain is about impermanence and publishing, not about "exfiltration".

I'm not sure which is more amusing, thinking that the FBI would give a hoot about b64-encoded data in the metadata of a transaction, or that this person wrote that comment on HN to try and seem cool. LOL, what, the FBI reached out to them (how?/why? nothing about this makes sense)?

The FBI is used to situations where they can issue targeted takedown orders when groups publish illegal information, but blockchain setups complicate the situation. How does a company "providing" an immutable blockchain stop providing illegal / unlicensed content? Does the fact that their blockchain contains illegal / unlicensed content mean the service is illegal to operate in the US? The alternative is having some sort of legal safe harbor where there is content that, in other circumstances, you would not be allowed to distribute but you can because you are on the blockchain.

Lots of distasteful outcomes possible.