Really?  With the exchange account in his name?  To withdraw $150K would require full KYC and docs.
To seize assets you require a warrant and that would require proving at least to a judge that there is sufficient cause.

Now if he had kept it all as BTC and sent them to a variety of encrypted and brainwallets that would be a lot harder.  Then again at least according to the press release Pirate was an idiot. I mean he lost the lion's share of what he stole.